I would suggest that if you are a brit, then it would be more prudent to keep all your quals JAA. I really like the idea of utilising the pound v's dollar to maximise your spending power. I know there are training organisations in the US where you can train for the JAA license. Perhaps that is the option. I have met a few guys who have tried to be smart with their cash and had various ICAO parts to a patchy license... but have actually come unstuck trying to amalgamate it all to get a job.
What ever you decide... goood luck.