An N-reg can be flown worldwide (outside the USA) on a license issued by the owner of the airspace. Ref: FAR 61.3.
However beware in Europe each State issues its own licence, there is no such thing as a JAA licence that allows you to fly a N reg in a JAA State other than the one where the licence was issued. As you can only hold one JAA licence that restricts you to UK.
A G-reg can be flown worldwide on any ICAO PPL, VFR only and if flying IFR then only outside controlled airspace. Ref ANO Art 26. (**)
But not IFR if its a JAA licence that is subject to JAR-FCL 1.175 ANO Schedule 8.