Except for the nighttrains you don’t need any reservations in UK AFAIK. You can just hop on the trains (show your pass to a staff member at the gates to acess the platforms).

Eurail planner contains a lot of WRONG information. So don’t trust that source about “reservation neccessary”.

I am wondering if I should drop either Bath or York as destinations? Would that make it easier? Train or schedule wise? I briefly lived in London years and years ago and my daughter has visited twice so we don’t need more than a day to day and a half in London. I visited Bath when I was a teenager and like it. Neither of us have been to York, Edinburgh or the Scottish Highlands.

You also need seat reservations on LNER trains which run on the route London → York → Newcastle → Edinburgh. The reservation is free though. 

https://www.lner.co.uk/travel-information/make-a-reservation/

I would book York → Edinburgh and Edinburgh → London through that site. There’s usually availability. If you’re travelling first class they’ll even bring you a hot meal and drinks.

On the GWR trains from London to Bath, for example, you can just get on board. If the train is full, you may not get a seat until someone gets off, but you’ll be allowed to stand or sit on your bag.

I think it is fine - one day for Bath and one for York.
I’ve been to both places and they are not so big, so you should be fine with a day in both of them.

@MartinM  Do you think the night train is worth the extra expense? I would want a sleeper compartment to insure we get some sleep but that adds a big expense.

Could you please provide a source that there is a reservation needed? I’ve used several LNER services last autumn and nobody asked me for a reservation...

P.S. It’s approximately 4 hours Edinburgh → London.

I haven’t been traveling by nighttrain in UK so can’t give a good response. But I’ve heard from other people that they have quite a good service. 
Probably some other members here can give a better answer on that.
If you can save on a hotel night, probably the expense is not too big ;-)

Sure:

https://www.lner.co.uk/support/faq/tickets-booking/national-rail-travel-vouchers/can-i-use-interrail-passes-on-lner-trains/​​​​​​​

Plus here on the Interrail site - see “Mandatory reservations required”:

https://www.interrail.eu/en/plan-your-trip/tips-and-tricks/trains-europe/trains-country/trains-great-britain 

Just for clarity

The situation in the UK is that many of the express/service operators offer an optional free reservation service. Although reservations are not mandatory except on Eurostar and sleepers it is highly advised you pop into a station before travel and reserve seats at the ticket office. There is no charge for this and ensures you have a seat and do not need to play musical chairs as you travel, as even if a seat is not reserved on departure, reservations are still sold as the train travels down the line, and that may be the seat you are in.

All ticket offices should be able to issue reservations for all operators.

UK trains have no limit on how many passengers can board so standing is often the case for unreserved passengers, even if they have a carriage available for passengers with no reservations.

Many regional services (with frequent stops) do not have a reservation service and they are simply add to your pass and travel.

Also note that many inspectors cannot yet read QR codes so they may look at your pass carefully to confirm validity, so do not worry that there may be a problem.

Do not miss York on your travels - It is a jewel, especially the rail museum by the station. If you are a true rail fan there is even a B&B/restaurant a few Km from York that has converted rail carriages for bedrooms.

https://thesidingshotel.co.uk/

It does not say on that page reservations are mandatory. It's just a request to book one.

@SamCritch thanks for the links, but the Interrail website is no trustful source (there is also written TPE with mandatory seat reservation, but that’s definitely not true) and on the LNER link it says only “please make” not must So it is not mandatory IMO.

(doesn’t make a huge difference as the reservations are so or so free, but had cases where I just dropped in the station and they couldn’t make a seat reservation anymore and then it was still no problem to board the train and also several ticket office clerks told me that the rail planner app is wrong with this information)

We have been trying to get this information corrected for more than a year now.

LNER (and several other operators) offers FREE optional online reservations and at busy times they are strongly recommended. At other times they are advised to avoid playing musical chairs as people with reservations join the train ( above seat indication is not reliable).

All LNER trains do have one second class unreservable carriage available on a first come basis, but that can be a bunfight as at starting stations they only allow boarding from a few minutes before departure, and at later stations there is every chance you will need to stand as those already standing will jump into seats as they are vacated.

I have booked LNER at Kings Cross as little as 15 minutes before departure for an earlier train after arrival from Eurostar and quicker than expected transfer.

Hello, I am confused as seeing contradictory information on whether or not I MUST make a reservation on LNER London to Edinburgh service.  I am a UK resident with interrail pass, returning to London on Eurostar on a May date and I expect to be in time for the last couple of connections to Edinburgh. I’d like to catch one of those trains in order to complete my journey home using the pass and not having to book a separate ticket.  Looking at interrail site, it says that Seat reservations required - see above.  I also see on Trains in Great Britain | Interrail.eu that seat reservations are mandatory on LNER.  I am trying to book this seat reservation - but when I log in to use my pass number, i get the error message in red saying that reservations are not available (but when not logged in - i get the page as attached)?  So first question why does it suggest that reservations can be booked, then when I log in they cannot be?   Second question, can I hop on the Edinburgh train without a seat reservation using my pass?   Thanks for your help here.

Yes, as written the posts above yours, there is NO mandatory reservation. Interrail doesn’t update its page regularly and there are a lot of wrong informations… 

Seat reservation is not mandatory, but on LNER still recommended. Book one free of charge here:

https://www.lner.co.uk/travel-information/make-a-reservation/

rvdborgt thank you.  I have tried that and i get this.  Is it because train schedule not confirmed yet?  

Quite probably. You can't book yet for 29 May on nationalrail.co.uk either.

Just to clarify - LNER policy is that there will be one unreservable carriage  on each train. Reservations are therefore optional but obviously once carriage is full of unreserved passengers you play musical seats in rest of train as reserved passengers get on as the train journeys north. Given that LNER hold passengers till about 10 mins before departure, there is then a Le Mans style race up the platform for a seat. 
https://www.lner.co.uk/travel-information/make-a-reservation/

Unreserved seating

There is a limited amount of seating available on a first come first served basis in coach C in Standard and coaches E or M in First Class. For those that do not get a seat there is space to stand. If you would like a seat we suggest you arrive in good time.

We usually reserve a seat for the second train from Kings Cross after Eurostar scheduled arrival, but if transfer is quicker than planned we simply pop into the ticket office and reserve for the earlier train, if delayed do the same for a later train. We have never had any problem booking a seat a few weeks before departure, nor on the day if we change our planned departure.

(You could also book seats for all possible trains as if you do not turn up there will be extra spare seats for unreserved passengers if train is full and standing.)

p.s. if you have a first class pass you can use the first class lounge at Kings Cross and access the platform via a different short route just before they open the main gates, getting to your reserved seat in a priority way.

@Yorkie @rvdborgt Thank you for replying.  In other cases, such as Eurostar or TGV in France, my understanding is that the seat reservation IS mandatory because there is a financial element to this i.e they want more money from the interrailer, there is a quota of seats, and once all the interrail seats are booked, you cannot travel on that train and have to go on a different one or buy a ticket.  So I am concerned that this is the case with LNER (so it’s kind of irrelevant if there is a carriage of unbooked seats that you could use as you might not even be allowed on the train) - is it more the case that the train companies want more to get more money out of the interrailer?  That’s why I’m asking.  I’m worried that I might turn up at Kings Cross without a reservation and then not be allowed on the train and then have to buy a full price ticket.  Thanks for your patience answering these questions.

It is not. LNER (and a few others) are sending incorrect data to the various timetables, specifying mandatory instead of recommended seat reservations.

That will not be the case. You can always board and stand or play musical chairs.

I agree with rvdborgt and in the UK it is extremely rare that passengers are prevented from boarding because of overcrowding - usually the passengers themselves take the decision to wait for the next train. 

There is a lot of negative hype about reservations and costs etc, but they are upfront and it is up to each of us to decide whether we want to pay for these trains requiring reservations or take the (usually) slower regional trains that have no reservation fees (or optional).

One small correction to your understanding - SNCF have a 2 tier reservation pricing policy for TGVs, they have a quota at 10 euro per seat then allow pass holders reservations at 20 euro per seat until the train is full. You can access this 2 tier system via the B-Europe service, but Interrail reservation service only offers the 20 euro option.

https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish

I will look into this. I will keep you guys updated. 

Please feel free to flag any issue you see in the website. 

Thanks!