You are exempt from wearing a belt if you are making deliveries where you are travelling less than 50 metres (e.g. milkmen and postmen), have a medical exemption certificate issued by your G.P., although I can't think of a medical condition worse than death; or are reversing your vehicle when you MAY remove it. I think Hackney carriage drivers are also exempt when plying for trade because they may have to jump out to assist a passenger getting in the cab. Yeah, like that's going to happen.
I don't think there's much to be said that hasn't already been said here. Except, don't crash in the first place, but then that's another thread entirely.