Few people pay with cash now anyway, I assume, or HAVE to pay with cash, seeing that most bus operators accept contactless and pensioners will have their OAP passes
True.
I'd go with a similar line to the trains.
Can you pay on your phone without overpaying? If so use that.
If you can't, use contactless (or your pass if you have one)
Only if you still can't, use cash
Though bus passengers are probably disproportionately represented in the number of people who only use cash.
FWIW it's well known here that I'm anti cash, but in this sort of situation I would support the other way round too - in a hypothetical emergency where using cash was the safest option, I would support that over card. It's more about greater good than personal preferences at this time.