Hi Mike
Are you aware that you can fly privately in the UK with an ICAO PPL(H) so I wouldn't see the point in spending £1500 on a JAA PPL(H) conversion, you could better put that money to your JAA CPL(H). Basically to start a JAA CPL(H) modular flying course you need 155 hours helicopter flight time and an ICAO PPL(H) you need to have a look in LASORS on the UK CAA's website to see the full requirements.
Advantages of a JAA licence over FAA or ICAO is it will allow you to fly in other JAA member states, not all JAA member states will allow you to operate with an ICAO PPL but the UK will (This only applies to private flights) I last spoke to the CAA on this matter about 2 years ago so give them a call to confirm this has not changed.
Good luck.