Maybe inadvisable, DFC, but you still haven't shown them to be illegal for private flights. Indeed the CAA published a paper (last year? what happened to it?) discussing two separate issues:
(a) approving approaches at airfields with only a FIS;
(b) making non-approved approaches illegal for all flights;
making it quite clear that at the moment the latter are not illegal. They wanted to tie (a) to (b) as a sweetener for (b).
Or are you suggesting that home-made NDB and VOR approaches are legal, but home-made GPS ones not? Not sure how that could be justified.
Tim