peter wrote:

Given the historic unreliability of London bus services, I would be
reluctant to put money into a machine until I knew for sure the bus
would come.

Fair point, though I think the OP referred to buses where you do pay the
driver, and many people board having been waiting for ages and only then
start faffing in their bag for their purse / wallet / Oyster rather than
having it ready in their hand.

Neil
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Neil Williams in Milton Keynes, UK. Put first name before the at to reply.