I'd be surprised if many ticket checks were done on the fast services between Waterloo and Surbiton. The trains look fairly packed when they turn up at Surbiton in the evening.
Beyond ticket checks, are there other ways to detect people sitting illegally in first class, assuming it's an illegal thing to do, rather than just breaking of a regularion?
I only sit in first class carriages if I know them to be declassified or I've bought an advance purchase ticket that's first class. Interesting to note have many people don't know when first class is declassified.