Yes, I've had drivers who have given discounts and with no ticket.
Situation; board bus with only about £1 in change, and a £5/10/20 note. Fare is £1.70.
Bus company policy is that if they can't break a note, a change voucher is printed. While I have seen one, I've never been issued one and I've said I'll take one many times! They ask if I have anything smaller, and if I say I only have 90p or whatever, the driver then says 'that'll do', take it and wave me on
Back in the dark days when I had to frequent bus transportation in Leeds, bus company policy seemed to be to tell you to sod off and get the next bus.