Personally speaking I have not had a problem with paying £20 and £50 to my GP for a signature, particularly if a few medical examinations were made, it's still less than a heating engineer charges for signing off a gas appliance, and my licence is invalid without it.
I assume Doctors/Practices must, in any event, have liability insurance for signing HGV and PCV medicals.
Of course, "fit to fly" is our responsibility, but all the GP was being asked to do, (with further consideration of medical history if necessary), was countersign to say that I met DVLA group 1 or 2 standards which are easy to check on their website under the "at a glance" pages.