Now I can fly G reg aircraft in the UK on my FAA licence but can I use my JAA medical as the medical, I was thinking yes been as its a ICAO state,, but its a little grey I supose
You cannot mix and match your medicals and certs. Your FAA certificate is only current with a current FR (FAR 61.56) and you need the medical issued by a FAA approved AME as per Part 67.4.
The only time a non FAA medical can be used is when the certificate is a validation based on a foreign pilot's license issued under FAR 61.75.