Subject to the disclaimer that while I'm involved with bus preservation I am not qualified to give "legal advice"
However, my understanding of the position is that if ALL of the following apply, then it is legal for someone with a car driving licence to drive a bus on the public highway
a) the bus is over 30 years old at the time
b) the driver is over 21 years old at the time
c) no more than 8 passengers are carried
d) this is not for hire and reward
That means that drivers at the many "bus running days" around the country do need to have full PCV entitlement (although if they are genuinely volunteering and the service is genuinely free, the driver CPC is not needed.)
These people are based in the midlands, and are the only organisation I know who offer a 'bus driver experience' day -
http://www.wheels.co.uk/coach/experiences.htm
There are preservation groups / museums and some operators of heritage vehicles around the country who generally welcome new volunteers, and in the case of the heritage vehicle operators, some opportunities for paid part time staff.
Most are unlikely to be in a position to fund PCV training, though, and even if you do have a PCV licence, most museums etc tend to have plenty of PCV drivers, and will expect volunteers to do their fair share of keeping the vehicles clean and healthy, and some of the less glamorous tasks involved in a running day (e.g. vehicle marshalling) as well.
Newcomers who just want to arrive at 10.00 once everything's been set up, get in to a bus that someone else has fuelled and cleaned, do a driving shift with a generous lunch break, then push off at 5.00 on the day of an event and not be seen again until next year's event may get a slightly chillier reception...
In many cases, conductors are in shorter supply. While it's not the completely unskilled job some people think it is, the training is less and more informal than for driving - although again, volunteers who play a part in getting things ready for an event are more likely to be given first choice of duties.
Where are you based? I might be able to suggest a preservation group / museum locally.