Think you will need to keep the money for a medical anyway as this could go the same way as automatic MOT, number plate and Insurance checks -
http://www.thamesvalley.police.uk/ne...anpr/index.htm
The Police can pull you over for a number of reasons, are they going to add this medical thing to a checkable database and any 'self-certified as fit' drivers will be targeted and will the Police have powers to enforce something like - 'you need a medical we are not convinced you are fit to drive'.