Well said IO540
It would be almost impossible to prevent 'N' reg aircraft from being operated in the UK.
To do so would open up a huge can of worms - for example, if an 'N' reg biz jet and its FAA qualified crew are deemed by some piece of ill thought out legislation to be inadequately maintained and flown by an underqualified crew, then so potentially is the United Airlines 747 that flies into Heathrow. It's a nonsense.
The threat to the UK IMC rating is altogether more real and we should all do whatever we can to prevent that from happening.