As with previous posters, my impression is inspectors are a lot less common these days than in the past. However I have had my ticket checked onboard a Stagecoach Wigan bus once since deregulation (probably this was when First were still running them, so before 2012).
In the past, I believe roving inspectors were just as much about detecting fiddles by bus crews as ticketing irregularities by passengers. For example conductors who helped to minimise wear-and-tear on their mechanical ticket machine by issuing some of the tickets from the "spare" machine they kept secreted away. Or early adopters of the recycling ethic, who "re-purposed" tickets retrieved from the Used Tickets box.
With electronic ticket machines, GPS vehicle tracking and constant CCTV surveillance (including in the driver's cab), is it harder for drivers to carry out worthwhile fiddles these days, and easier to detect these just by sitting in an office looking at spreadsheets or CCTV images?
An account of life on the buses with Glasgow Corporation back in the day described how, as well as the usual uniformed inspectors, they had plain-clothes travelling inspectors surreptitiously keeping an eye on crews and known locally as "The Gestapo". I also seem to recall reading that members of the Gestapo were all cut from the same mould - ex-sergeant-major types with bristly moustache and tweed jacket, so not that hard to spot when about their clandestine duties, once you knew who to look out for.