I travelled First Class with FCC from Farringdon to Luton Airport
yesterday evening.

Got into the rear compartment to find that it was almost full but I got
the last seat (Standard was full). Just after we departed, two FCC
guys (presumably RPIs) came into the compartment and asked to view all
First Clas tickets. At this point, several people tried to make for
the door but were prevented from doing so. In the end, 7 people were
penalty fared between Farringdon and West Hampstead Thameslink for
sitting in First with a Standard ticket. They all got off at West
Hampstead, where 3 more people came into the compartment. Immediately,
the RPIs came in and all 3 got £20 penalty fare notices. Another 2
boarded at Harpenden - one had a First Class ticket and the other was
penalty fared. A real field day for these RPIs!

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
checks. However, I've noticed that FCC RPIs are far more common now
than previously on Thameslink, where there were hardly any checks at
all north of the Thames.