In short; NO, you will not be refused travel in UK due to a train being full. The only exception to this is the handful of overnight sleeper trains that require reservations.

Travelling without reservations is normal in UK, many do it all the time, trains without any seats for a whole journey are very unusual, on routes such as this you are only likely to be forced to stand when there has been serious disruption.

Train cancellations are higher than usual at present due to a number of reasons, it is best to check at the train operators website for disruptions, in this case www.scotrail.co.uk

If you want seat reservations they can be made for free by creating an account on www.gwr.com and using the Bookings - seat reservation option. when asked select anytime single or similar (NOT Advance) as ticket type for reservation, it is also fine for interrail pass. Only select 1st class if you have a 1st class interrail pass.

Cost for seat reservation is always 0, if asked for payment you are in ticket buying not reservation section.

Thank you very much kind sir or madam. You really calmed me. I tried the gwr way of seat reservation but some trains didn’t give me specific seats. It only said that the booking is completed. So I just wanted to be sure.

Thank you again and have a nice day!

That happens on certain trains/routes that don’t offer specific seat reservations, usually secondary routes.

A quirk of the UK system is that to sell their cheaper tickets that are restricted to specific trains they use reservations not train numbers, on trains that do not provide seat reservations these blank reservations are used to restrict these fares to individual trains. For anyone with unrestricted tickets they are basically pointless as the seats on the train will be unreserved throughout. If you haven’t been issued a specific seat then nobody else will have either.

Edinburgh - Inverness normally has seat reservations, it may be with their current degraded timetable they have suspended them.

You can also make the RES-or rather-have it done for you-at all main stations @ counter-in a few secs and also for free.

Most of the IC-trains that have many RES seats-there is 1 car-often the furthest away from stairs etc-wholle UNreserved, but I dk if that is policy in general or just 1 or 2 operators