I'm not actually a Dr, but a consultant Engineer - I don't fart unless I'm paid for it. (we're nearly as bad as lawyers)

I therefore have some sympathy with GP's being asked to perform a task beyond the scope set out in their contract. 'Some' being the operative word in that sentence... I'm just trying to see the flip side.
I agree its complete shambles - essentially in Scotland it could be your GP that decides your renewal/grant rather than the police, which is most certainly not the point of the guidance or law (I believe.) That decision rests with the police and they should not be trying to divest that responsibility.
If I were a Dr and the patient was well known to me then I would agree that £20 is pretty reasonable given that its two or three tick boxes and a quick refresh read of the notes. If I barely know the person (which would be unfortunate for those not ill often, including myself - seen the Dr about 6 times in my entire life! or recently moved) then it may take me longer and I may contact the patient to make an appointment to discuss. If the patient makes the appointment then I'm sure this would be encompassed by the NHS and no charge levied, like being called in to discuss test results. Again, I think £20- £50 is pretty reasonable as a punter, but I realise that based on that price list it could be a lot more than we want...
In either case I would be tempted not tick the first (or last box, I cant remember which) that states that in my opinion the person is safe to hold a shotgun/firearm and return with a note that its the polices responsibility to determine not mine. That whole question gets my liability/litigation senses tingling... The other two questions are factual - Does the patient suffer from one of the notifiable conditions, have you placed a marker on their records. Easy and pretty safe.