Ive been travelling in the UK on Britrail for many years. Only ever been asked for proof of residence once, on what was Virgin East Coast, which was the one time I'd left my docs behind. I just about managed to do it by showing a lot of non-UK utility and banking apps on my phone. I usually carried a copy of my income tax return and contract (finding something with an address in it was not easy). These days I carry a copy of my non UK residence card and my non UK passport (the two are from different countries), although in recent years I've not been asked for anything. I always wondered how things would go for the rail employee trying to read the documents in a language they'd almost certainly not know! My other half has occasionally been asked, but their mainland European passport has always satisfied the asker.
The new digital version of the ticket, very similar to the UK flexible season tickets from what I can see, is very reliable and impossible to cheat (which was quite possible with the old paper based version).
Britrail used to be phenomally good value, even in first class, and with occasional free extra day offers it was genuinely cheap. They've steadily been putting the rates up over the last few years and these days it's more that it offers total flexibility (ie you can treat any train at any time as if you have a walk-up ticket and change your routing at any time) that makes it worth having, and even then you really need to be making a few fairly complex long distance journeys over several days. Detailed knowledge of the GB beennetwork has always really needed to make the most of it.
