You are understandably confused. In the UK reservations are free and optional on all trains offering reservations. Interrail planner is wrong when it says reservations are mandatory. You can travel on the direct train with no problem.

However because the UK do allow unreserved passengers to travel the popular trains are often over-full and unreserved seats can be scarce and you may end up standing or playing musical chairs as passengers with reservations board during the journey.

Some booking agencies do charge a fee but you can get a free reservation at any station with a ticket office. Also GWR offer a reservation service for most UK services.

Thank you, Yorkie, for the  prompt reply and encouraging information. Then I'll take the direct train, in hope, I get the Eurostar from Brussels to St. Pancras on time. It should arrive at 7 pm and brisk walk to Euston should suffice, don't you think, to catch the 19.46 to LLS? 

You'll even have time for a quick bite :)

The only concern would be the time taken to get out of Eurostar terminal, but it is only 15 min walk, so an on - time arrival should be fine. There is another direct train an hour later.

Yes, that'll be nice... thank you again

I know, that's the one I would have waited for at Crewe,  had I followed interrail reservation info before you enlightened me.

Might I even be able to catch an earlier Eurostar? Probably not, if the reservation (Standard Premier) is for the 17.56 train to UK, arriving at Brussels midi at 15.35 from Frankfurt a. Main. Fingers x that my original plan is doable. 

It wouldn’t do any harm to ask if you arrive early. They do offer exchanges at 15 euro, but if there are spare seats you never know.

I assume you have a first class pass for standard premier. If so enjoy travel in the UK - you should be able to use the lounges if you have to wait.

In that case, you'll also have food on board the Avanti service:

https://www.avantiwestcoast.co.uk/travel-information/onboard/first-class/first-class-menus

Yes, 1st class, with all the trimmings

On Friday 14th I'm leaving from LLS for Preston. As this journey only costs a few £s I'm not using my interrail card for a second day (I've got the 4day in 1 month). Would I still be able to use the lounge at the station before travelling?

you need a active travelday to use the Lounge with your pass :) 

Or a 1st class ticket from LLS to Preston :) 

Ah, that's reasonable, I suppose.  Thanks for letting me in on these tricks and hacks. 

The 1st class lounges in UK stations are run by individual train companies, you need to be travelling with that company or at least have a ticket valid for travelling with them.

Having a first class interrail pass would be eligible though, wouldn't it?

As long as it is activated for travel on that day.

Thank you, I understand now.