The Brits have been sneaky because even though they validate your licence they then proceed to limit you to the very minimum of only day VFR private flights so there is no wiggle room how ever much your ticket would nomally allow you to do unless its a JAR license. But they will let you to fly a microlight.
Using Validations in a third country airspace gets alot more envolved.
As soon as your in someone elses airspace you have UK ANO and the airspaces "owners" ANO to comply with. It not an automatic right that you can fly G reg anywhere you like with a FAA ticket just because the UK ANO says its OK. yes your 100% bullet proof in the UK FIR outside that you better do your home work.
I will leave it to Peter etc to give you a more infomed answer to that though.