AGA RPIs (can't speak for the guards on some of the more rural services, as that's a whole different kettle of fish) won't issue PFs or often even check services where first class is declassified.
By their own admission, doing checks on some of the stoppers that are declassified are a pain - and I guess they deem it more practical to do blocks at stations themselves.
As such, once they realised they were working a train that didn't have first class, chances are they soon alighted and took another service instead?
I've had many RPIs genuinely not know what service they're working, and get confused when consulting their own timetable. If a train is late, it's even more confusing. But, in 99% of cases just telling them sorts it out.
Only once have I seen people issued PFs and them being immediately refunded there and then once it became clear there was no advertised first class (this was with FCC, not AGA, mind).
I know it's a can of worms, but I am pretty certain all the TOCs operating services into London will have the same policy. Far better to concentrate on services with first class and avoid any hassles, given the fact RPIs are already spread pretty thin.
