National Express West Midlands certainly had them when I lived in Birmingham 2000-2015. The north-bound stops at 6 Ways Erdington were frequently used as a location for on-board inspections, and I'm pretty sure a drug sniffer dog would occassionally embark on a quick tour of both decks ......
It was really a combination of revenue protection and a high-visibility response to anti-social behaviour.
Similar era. Unlike other inspections I have come across (eg London, Stagecoach South East), the bus stopped, someone (Inspector, Police Office) would go upstairs and ask passengers to have their tickets and passes ready. There would be at least two Inspectors, but usually more, checking tickets and others - usually police - waiting for anyone trying to do a runner. It was a serious operation. I was stopped just because I happened to be getting off - my pass had already been checked (6 ways going south).
They tended to check at places where several routes pass. I can remember Acocks Green, Bearwood and Witton Cemetery. There might be buses behind waiting to be checked.
It was a minor inconvenience but judging by the numbers of passengers I used to see disembarking and lining up, it got results.
Maybe i can kind of understand on TFL buses which have rear doors where people might sneak on without the driver seeing. Also the drivers on TFL buses rarely seem to be properly paying attention to any ticket you show them.
Some sneak on when those with prams or pushchairs get on via the middle doors. I try and avoid the LT class but I understood the rear doors were not used any more (thanks, Boris). Otherwise most tickets and passes are surely checked by the 'Beep machine' (whatever it is called), only interlopers like me from outside the Capital need to be checked. If people think their ticket will be checked, more will buy them. Not far away there are three consecutive stations, schools at the two end ones, journey time 7 minutes. Some journeys are pretty packed (and not just with pupils), none have gates, staffing is nil or part time, travelers tend to exit via side entrances, not the booking hall. I reckon there are quite a few who haven't paid. Little incentive to do so.