HillerBee is right, HOWEVER..
You need to consider a couple of things here..
1. JAA License or FAA license and why?
2. Flight hours (experience) to get your career started.
What I do know, is that it's gonna be a hard time getting a job as a commercial helodriver in the UK, or Europe for that matter, with a couple of hundred flight hours.
This is where the FAA license comes in to play.
Because, on a J1 Study visa in the US, you are allowed to stay for about 24 months doing your training and hopefully getting some flight experience as a flight instructor after. (Yes, flight instructors are hired with less than 200 Hr's experience in the US!)
Then you enroll with a british ground school ie. for your full JAA training, there is no "conversion", however you will already have a good foundation from the FAA training plus lots of flight hours to build on now!
You can actually do the JAA CPL-H fligth test with Helicopter Adventures Inc. in FL.
Last but not least.. Price! A dual flight lesson in the US (east coast), in a Schweizer 300 CB/i is about $205 Hr these days.. What is it in the UK?? Lots more I'm sure!
Good Luck!
WingRotor