Isn't it that the validation rules DO NOT APPLY to non EU operators (pilots), who are deemed already validated? So a US citizen coming to the UK can fly a G reg with no formality. Also a brit living outside the EU is not required to have a validation.
For example, if I were a permanent resident of say Brazil and I came back to the UK then I can fly a G reg or N reg without having an EASA licence. If I moved back to the UK then I would require one.