I would be very careful using a JAA license anywhere in Europe. FAR 61.3 states "However, when the aircraft is operated within a foreign country, a current pilot license issued by the country in which the aircraft is operated may be used."
As Mercuray correctly points out, JAA member States accept each other's licenses. A license issued under JAR-FCL 1 is still issued by a specific State and therefore only acceptable within that territory. It would be different if the license were issued by the a single common body such as JAA/EASA.
I'm sure an insurance company will be more than happy to use a strict interpretation to avoid having to cough up...
Erik.