Perhaps it's not as bad nowadays as it was. Bus operators now rather cheekily charge fares rounded to the nearest 5p or 10p and increase by this amount even when this is way in excess of inflation. There is now a sizeable proportion of travellers who do not pay at all to travel, though often it was those over-60s who kept a bus driver in a good supply of small change. The mass uptake of great value all-day tickets within urban areas, that can be purchased from the driver, has limited per person cash transactions each day. There are a few operators who've always opted for an exact fare system, offering no change. These operators haven't increased in number though.
The reason I've been pondering the whole 'giving change' thing came following a trip within the LEYTR area that a friend and I undertook. We caught a bus service where the fare was £1.85. We were only to make one journey on that operator's service that day so an adult single was all that we needed. I boarded first and gave the driver two £1 coins as I didn't have £1.85 exactly. The driver asked if I had the 5p - I did - and duly handed it over, to then receive a 20p coin as my change.
Unbeknown to me, when my friend boarded next, he too offered two £1 coins. Like myself, the driver asked him if he had the 5p. Sadly, he didn't, so the driver said he was unable to give him all his change and presented him with a 10p coin. My friend was unaware that seconds earlier I'd given this driver a 5p coin, as requested, and the driver chose not to give this back out.
I made my friend aware of this and he wasn't too bothered that he'd paid £1.90. I wouldn't have been too bothered either, had the fare been advertised as £1.90 (or £1.80!). We were both aware that no operator has any legal obligation to offer change, but virtually all do so in order to attract patronage, rather than turn it away - something Nottingham City Transport, Lothian Buses et al must surely do to some extent by no offering change.
So the driver was 5p in the black. He may have dutifully paid in that amount over at the end of his shift, but I bet he didn't. It was also pretty obvious that my friend and I were travelling together. The driver just didn't care that we would work out what he'd done: needlessly keeping the 5p.
This is a relatively minor story and while only 5p is involved here, there are occasions where 8 kids all want to buy £2.60 returns and each present the driver with a £10 note. If the driver doesn't have any £5 notes, a total of 48 coins need giving out in this batch of transactions. That's why the way forward is something I've only recently come across: the No Change Voucher.
Municipal Grimsby-Cleethorpes Transport never had anything like this when I was growing up, neither did NBC-operated RoadCar; that was until the latter acquired the Almex A90. A No Change ticket could then be issued to a passenger where the driver could not offer sufficient change. This ticket would then effectively be worth cash to any RoadCar driver and the passenger had the confidence that he/she wasn't 'being done' or had to make their way to the company's HQ with a hand-written IOU from the driver. The irony here is that sometimes this would mean paying to travel on one of the operator's buses.
That's not to say these vouchers don't have their pitfalls. I once caught the Oxford Tube at Marble Arch and every single one of the 6 passengers in front of me were given a No Change Voucher as they boarded. They were all headed to Oxford and the driver told them to approach any Stagecoach in Oxfordshire driver there for the change. This wouldn't have worked if they were London-bound, unless another Oxford Tube driver was able to oblige. That's assuming there was one there.
In a non-aggressive manner, the No Change Voucher does reinforce to passengers the onus they have of providing the correct change whenever possible. Like Peter Kay's Brian Potter (Phoenix Nights) said about garlic bread, "I've seen it. It's the future!"