I have been traveling from Swansea to Paddigton on a Monday morning and home on Friday afternoon using FGW for months and always travel in First Class, because I book in advanced its £98 on the way up and £38 on the way back, I think that's pretty good for First Class.
If I turn up at the station on the morning of travel it will cost me £114 single and £42 for my return in Standards Class, booking in advance not only gets me cheaper travel but I get to travel in First Class.
I have seen many people being caught out by being in First Class when they shouldn't have, foreign people on their holiday and not realizing to people blatantly trying it on, they all get treated the same, pay full price on the day First Class rates which is a few hundred quid or move into Standard Class, everyone moves, I have not seen anyone getting a cheap upgrade that has been mentioned.
Personally I wouldn't sit in First Class if I hadn't paid for it, I would not like the embarrassment of being asked to move.