Megabus is a brand of Stagecoach- so all the buses run on Megabus routes are Stagecoach owned and operated. There may be occasions where Stagecoach may have to hire in someone else due to a failure, but only in exceptional circumstances as of course normal Stagecoach livery buses can fill in instead...
I don't use Megabus as much as I used to, but it was fun during the early years with the ex HK Leyland Olympians or if they didn't work... a bog standard Trident, a few were specially tachograph fitted for this purpose!
I remember bashing Tridents to Oxford, Birmingham and Brighton. Even had a Volvo Olly as a duplicate on the overnight run to Glasgow (though sadly it only made it to Keele services, but that was due to no fault of the bus itself).
I am have even bashed an Alexander PS on a Megabus working when the scheduled bus failed just south of Oxford, so Oxford depot sent the PS out to get us off the motorway to Lewknor in order to board the next Oxford Tube to complete the journey!
Fun times, sadly the single deck coach types now operated on the Megabus routes don't have the same appeal...