Had a ticket check on a London bus last week, supported by police. Plain clothes boarded at one stop and then started checking, police were positioned at the next stop. They didn't get to me by the time the bus arrived at the next stop, and for some reason nobody was covering the back door (it was a Boris routemaster). I was getting off to change buses anyway - so I did and then turned back and asked if they wanted to check my ticket.
(I assume the RPIs then crossed the road and took another bus back to the previous stop, there may have been police on the opposite side too.)
A funny incident on the Croydon tram - two scrotes boarded and started loudly boasting to each other about how they never pay for the tram, next stop RPIs boarded. But luckily for them, someone standing by the door hadn't paid either and he tried to slip past the RPIs unsuccessfully. While the RPIs were occupied with blocking that fare evader's exit, the scrotes did manage to slip past, tapped their bank cards on the readers and got back on... but the RPIs got off to process the caught passenger and didn't check anyone else!