First: It is NOT the standard for an HGV, it is professional driving such as a mini-cab or minibus. With an HGV medical there are all sorts of tests required. Not so with an NPPL.
Second: this page from the BMA expalins why GPs might charge for a signature:
BMA - Why GPs sometimes charge fees
The GP, in order to cover their legal backside, must actually look through your medical history. Now, since they have four minute (or whatever) slots, that means taking a little time.
A good level to start is the signature on a passport photo - which comes in at about £20. Of course, that does not need a full examination of one's medical history. But if - like me - you never see your GP, then a full examination of the paperwrk doesn't take long!
Of course, if your medical history is more complex, then the doctor might take longer to read your file - and just like a more complex repair to your washine machine costs more, so will that.
Of course, it could be argued that your GP will know your medical history by heart if you are a frequent visitor!
Unfortunately (or fortunately) we no longer have the family GP for life.
My mother was a GP all her life in the same practice and she knew the medical histories of about 6,000 patients - and the links to their families (close and extended) off by heart, it seemed. So she probably wouldn't have felt the need to examine your records closely.
Every time my wife goes to our local GP surgery, she gets a different GP (usually a trainee!), so no prior knowledge of her or her family's history.
ps. There is a new revised NPPL medical form published the other day.