Have seen VOSA checking coachs plenty of times but they are more selective of when they do it out of consideration for passengers in fact the old PSV test was very stringent &a vehicle could fail for dusty seats!.Have seen recently stretch limos prohibited&taken away on a lowloader 
Sometimes they will do checks at all day events to check how the drivers are getting their proper rest period. Many times I have been sent up to collect passengers who have been stranded by another companies dodgy activities.
They also turn up and check rail replacement buses on occasions and send the odd one home again.
i was pulled twice when i was coach driving by what we used to know as the ministry man , colin wadilove was his name 
however on both occasions i was on contract hire ( school swimming baths run ) rather than private hire and from what i remember the rules were/are different .
i was " done " for excessive smoke the first time and that resulted in MOT retest , the second time was rear emergency door buzzer not working but if memory serves that was just a warning due to me being on contract hire …
im sure thats right but it was a long time ago , 20yrs or more , and some of my memory has fallen off 
mrx:
They do. I’ve seen them stopping hen night limos as well. Never seen them with a service bus tho.
assuming they haven’t improved since i was a student, it would probably take too long to write out oll the defects on a service bus
They sometimes go along all the buses/coaches at the Millennium stadium on big event days. Empty vehicles=no upset punters!
Cheers
Paul
They stop everything at pont abram services at the end m4 ,vosa like it there when the sun is out…
You can imagine “disgusted of Tunbridge Wells” happily sitting on the motorway for two hours after the electric ■■■ terrorist alert but getting stroppy after being told the coach they are on is a potential death trap and it’s two hours for a replacement to arrive.
as i a coach driver i have it on good authority they wont stop you en route i.e pull you into a check site with passengers on as they dont want 40 odd passengers milling around or wanting to get of the coach for a ciggie while they do what they have to do. if they have to put the driver or coach of the road that gives them a big headache as in what to do with the passengers until other arrangments are made for the passengers.
I have been stopped en route while running empty a few times.so when im running enpty during the night i close the curtains and put the night lights on that way they are not sure if im empty.
vosa can be found doing spot check on coaches at all the msa sites doing checks on coaches and at all the big venues around the country football grounds ect infact anywhere coaches are parked up.
and they are just as vigilant with coaches as they are with trucks
Might well have changed by now,but I was pulled in on the M2/A2 roundabout a few years ago heading for Dover with a full load on.Safety check-1st aid kit,fire extinguisher,emergency exits unlocked.Quick tacho check and a look at general vehicle condition.I was polite,they were efficient-but there was no doubt that anything out of place would have been my problem to sort re:passengers etc.