Bring all succh aircraft onto the UK register (or export them if they are not fit to be here). It will be safer for all!
Just my 2cents here, I did the JAR-FCL PPL course AND I did the FAA-PPL course.
I have to tell you, the FAA wants to see POSITIVE control on a plane, I became a better pilot because of the FAA training. Same on the IR. While it's true that you have to study more nonsense on the JAR-FCL IR than the FAA-IR, I'm absolutely sure that FAA trained pilots are BETTER pilots than JAR-FCL ones.
The practical part, meaning to fly the plane is just much harder and intense on the FAA part, any pilot who did the FAA training will agree that it's no joke like many JAR-FCL pilots think it is.
Again, I know, I've done both. It will be safer if you train all UK pilots to FAA standards - that is my opinion.