DFC,
Approved GPSs have RAIM warning, which is akin to the warning flag on your VOR/ILS display.
That is one of the key differences between handhelds and installed approved GPSs.
You can read up more about this
here.
The CAA does approve sets for en-route navigation but not approaches, so not sure if they are too worried about the technological side of the matter.
Could it be the time money and effort required to get the GPS approaches sorted.
The UK system does not allow IAPs without an approach controller. These creatures are fairly thinly spread over the places that do have IAPs. Places that could be most interested in GPS approaches are those that don't currently have any approach and may be unlikely be able to afford the bells and whistles required to set up the entire entourage for an IAP.
I suspect there is more to it than meets the eye from our perspective.
FD