Thank you chaps, good stuff.
As I wrote, I would have called up ATC but I wonder if they would have refused even though the reason is not connected with traffic. Presumably the CAA have instructed them to refuse non-G-reg. The really funny thing is that the Jepp approach plate says "VMC" but doesn't say "G-reg only" so they may well get nasty foreigners with dodgy Mongolian registrations turning up and asking for the approach
Fuji - the reason would be simply to make sure the GPS is correctly sequencing through the approach. This can't be tested just anywhere else, because while the auto-sequencing from enroute to terminal mode will happen within 30nm of the FAF or so (so this can be tested some distance away) the auto-sequencing from terminal to approach mode happens only very close to the final approach track.
It's not like practicing some VOR approach using another VOR 100 miles away, which is what people normally do.
However, there may be another way: selecting the conventional (e.g. NDB/DME) approach out of the database, and then flying that approach officially, should generate the same sequencing. I don't think the GPS differentiates between a "proper" GPS approach and an overlay of a conventional one. But for some reason I have not managed to get the final sequence in this to work, even though (I have verified) the GPS is correctly configured for the full US-style GPS approaches. Time to do some digging perhaps