You forgot to post the most crucial: WHAT type of pass-as profile says youre UK.
The USA friend can simply use an EUrail and this has no restrictions.
YOU, if UK, have to use INterRail (there seems a common misunderstanding among many a Brit that this is ceased with Brexit-complete nonsense) and tat -in the mobile version-only will allow you on MAX 2 traveldays to ALSO use in GB. Officially to reach a port, airport or trainport (Pancras for €*), but that is often not really checked-but it can. So yes-you got it right-you have to buy, probably at much higehr rates, those trips in GB over this limit. And do not forget all the REServations in foreign outlandish areas-where needed.
Yes I would get an interail pass. I’ve been trying to message them or call them but can’t find a number 
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Would it be cheaper to book early and just pay each journey separately
Before you do anything I will explain the In/Out rule so you can decide on your plans:
With an IR Global pass of any length you can use your pass in your home country on 2 travel days in any way that you wish on partner operators.
In the UK this includes almost all trains including Eurostar.
You will use one of these going from London, so as long as you can complete your post ferry journey on trains leaving before 2259 you can use a second travel day. If not you will simply have to pay for trains on the second day. If you are planning to take a few days for this journey you can choose which day you use your pass for maximum benefit. Bear in mind you can use your first day for all trains in that travel day, both in the UK and beyond E*.
These days are optional and come from your travel days allowance,
Your friend’s Eurail pass is not subject to this rule so has unlimited use in the UK.
There are no blacked out trains e.g. at peak times, with your pass.
In the UK almost all trains are either optional reservations or no reservations possible and are free from the operators. However I strongly recommend you make reservations for the express trains you plan on using (LNER in particular) as they will reserve all seats except one second class coach if it is busy.
A travel day runs from 0000 to 2359 CET (2300 to 2259 GMT), and includes all trains scheduled to depart by 2359, irrespective of arrival time.
APC rail are a Canadian and UK based seller that has been in business for many years, I don’t know what reviews you have read but as far as I am aware they are fine for selling passes, I have used them from the UK a number of times and had no issues.
However they also sell reservations for UK trains, DO NOT use them for this, they charge a fee for reservations that you can get for free at UK rail stations or from train operator GWR.com online.
APC rail are a Canadian and UK based seller that has been in business for many years, I don’t know what reviews you have read but as far as I am aware they are fine for selling passes, I have used them from the UK a number of times and had no issues.
However they also sell reservations for UK trains, DO NOT use them for this, they charge a fee for reservations that you can get for free at UK rail stations or from train operator GWR.com online.
Why would anybody wanting a mobile pass buy from anybody but Interrail or Eurail website? Best price, Instant e-mail confirmation and no third party involved?
DO NOT BUY PASSES FROM THIS COMPANY.
