Well, I have emailed the flying school who kindly sent me various PDF brochures and the following explanation:
Both Licenses are accepted and thee is no conversion needed on return. The JAR Compliant PPL is a FAA PPL with UK aviation law and RT procedures ground school and knowledge test. The JAA PPL is a PPL issued from the CAA. The only difference is that the JAR Compliant PPL is easy to maintain.
So - would going down the 'JAR Compliant Route' (AKA FAA PPL + extras) be a bad move? I do like the fact that it works out cheaper, being a poor student!