I read the DVSA document (an agency I'd not heard of until today) but none of the worked examples feature charity work or care homes.
The nearest approximation I could find was the farm labourer who mainly picks crops but also ferries the other labourers from field to field, on behalf of the farmer. CPC required.
I'm not a lawyer but, if I was, I would be an awkward s0d and suggest that ferrying the elderly around is a specialist skill and CPC should be required.
Some clarification of the hite and reward side would help too. The minibus service is part of the care home package and has clearly been paid for: it's just that they haven't paid to get on that specific A-B ride, like a bus passenger does, or upon alighting, as a taxi passenger does.