I too have never heard of a deposit.
As Gertrude notes, cancellation / 'no-show' charges are commonly found in the small print of many flying school's rules ... but, at least in my experience, they are rarely enforced; at least, not against one-time offenders.
I have never heard of any flying school insisting that weather is flyable when a pilot thinks otherwise. Besides it being mere common sense that no pilot (especially an inexperienced one) should be bullied into flying in weather that he or she is uncomfortable with, there would be obvious liability exposure in the event of an accident ("I told them that I couldn't handle that 30 kt crosswind; but they said that I had to fly or I would forfeit my deposit").
a nominal sum of about £50
Being generally 'cheap' (I'd like to say "frugal") by nature, I don't consider £50 to be all that nominal!
