A lot of the GPs I know are convinced that they are being asked to vouch, in some way, for the applicant's suitability.
I would think, in the majority of situations, the high fee are because of this and not profiteering. Most really don't want to be doing these forms which is something the BMA overlooked when agreeing that GPs might (I say might as they can't be compelled to) help out. As
@25 Sharps says, that leads to them pricing at a level intended to put people off.
Many employed GPs don't keep their private fees (the surgery gets them) so have no vested interest in charging for financial gain.
That's not to say some don't profiteer or behave improperly (I remember the chap on here who was told his GP conscientiously objected but could have their conscience calmed with enough money) but I fear the fundamental issue is a reluctance to be involved borne out of a mistaken impression that they are giving a professional opinion.
I imagine the Police could be clearer that they are not seeking that, only a confirmation that the declaration is correct, but wonder if they would rather have the ambiguity there, as to make clear that it is not the GPs responsibility to assess the applicant would highlight it being theirs.
As an aside, once the initial review is done and the marker placed on the system, there is no reason for future requests to the GP.