Thinking about it, I got caught out in Birmingham. Got a bus from the city centre to see my brother and was directed to put my money into a funnel type collector.
I think I gave £2 for a fare of about £1.70/80ish (had no idea how much it would be before) and didn't realise it didn't give any change until too late. I left it as it was minimal. I guess if there is a set system of no change given then everyone knows the score, well, maybe not non locals but you catch my drift.
Would have been good to have had some kind of notice though so at least you know.