Going back 20 years when I attended St Brendans College in Brislington, the college transport was contracted to South Gloucestershire Bus and Coach (with the Home To School Transport team at Bristol City Council handling the contract) and billed as a 'coach' service but the company often used buses, particularly on days when they had National Express dupe work on.
SGBC gave up St Brendans in July 2007 and by that time they were only operating the route from my area (Yate) which was contracted for something like a 35 seater coach but in reality, was using a midi bus in the morning which then went on to do a local service (511 if I remember correctly) and whatever bus or coach they had available in the afternoon. At this point, SGBC were beginning to ramp up their NX involvement having recently gained their first scheduled diagrams (weekend extras on the 040) and their bus operations were about to be or had just started to be taken over by Wessex Connect.
Separately I remember once riding on a rail replacement service with that company between Bristol Parkway and Yate which was using a bus rather than a coach and the driver said something like "Its got 4 wheels and seats..it does the job" and I also once rode a Red Arrow between Derby and Nottingham which was using a conventional Trent Barton Bus rather than a coach.
Also remember once seeing conventional buses on the former Greenline X7 (London to Bracknell) route and the London to Stevenage route (forget the number).