Never heard of deposits. I have heard of cancellation charges, which theoretically are applied to people who book planes and simply fail to turn up without phoning the club.
BUT the pilot must be able to cancel for any legitimate reason, such as: "I didn't sleep very well last night, so I'm not convinced I'm safe to fly, so it's probably better if I stay on the ground". There's no way anyone else can make this judgement; there's nothing objective that a doctor could see or measure to support your claim to have the deposit returned. Being charged £50 for this will, as you say, end up with people flying in marginal conditions they're not entirely happy with, and in due course this could result in bent aeroplanes and dead people.
I would recommend that you do not fly with anyone who asks for a booking deposit.