I suspect that this kind of thing (Standard people sitting in First on
suburban lines in London) is very common, where there are few ticket

It's probably more common because standard is full, and whilst it's
admirable that the RPI's did their duty I feel the train company would be
better off concentrating on ensuring the size of train matches the number
of those travelling. Although maybe the overcrowding is deliberate to
encourage people to buy first class?

I'm on an annual standard and wouldn't sit in first unless I had
permission, but it is kind of galling to see empty seats, especially as I'm
the last stop before London so it's unlikely anyone else would need that
seat.

Dave