Having recently discussed this issue with someone in a similar situation, both in discussions with the CAA and the BMAA I can clarify this further - you can fly any aircraft in the UK that the FAA licence allowed you to fly - but be warned - a JAR SEP does not cover the same aircraft as the FAA PPC, because of the different ways a/c in the US are classified vs the UK.
For instance as I understand it an FAA PPC does not let you fly UAs (correct me if I am wrong!)....in the situation I was privvy to, the aircraft is classied as a Microlight in the UK, but an SEL in the US - so in this case the pilot *could* fly the microlight on his FAA PPC perfectly legally...in fact he had an FAA CPL but situation is the same....
Hope this helps,
FF