From the experience of nearly 40 years in aviation I know that the CAA medics are a rather cautious bunch, perhaps not unreasonably, and that GPs signing pilot medicals don't really have a clue, but seem to be scared of the consequences of signing anybody off who hasn't got apparently perfect health.
The documents for the GP state very clearly that the GP is
not being asked to assess whether you are medically safe to fly. S/he is being asked whether they would pass you to drive HGVs. The CAA are deliberately trying to avoid GPs who don't know anything about flying denying people the right to take passengers unnecessarily ... according to the documents, at least.
I currently have my medical suspended by the CAA for cardiac reasons. However my GP is perfectly happy to sign the NPPL declaration so I can take passengers.