My info, coming from a DPE yesterday (in the USA but he travels to europe for checkrides) and I am sure it's correct is that they all have to register with the FAA, in NY I seem to recall, before going abroad to do checkrides. It could be a TSA matter; I didn't ask. I didn't get the impression it is a "permission" which might be refused.
The only advantage of doing a checkride in another european country is if the UK-based N-reg owner somehow doesn't meet the DfT criteria (say, he is in a group of 6 owners, or he isn't a Director, etc). It's all a lot of hassle, and a DPE for a few days will cost you a few grand.
I reckon it's better just to go to the USA.