Hello mate, I did what you are proposing. I flew a CRJ in the USA and converted to JAA then got a job in the UK. Most of what is said below is correct ...however.
1. Get a JAA/EASA Medical Class 1 first before you start (it's not a guarantee pass), the CAA website will list overseas JAA/EASA Doctors BUT for your FIRST Class 1 you will have to come to Gatwick. It used to be called an extended renewal as they gave you credit for a foreign medical.
2. As you're already flying a transport cat airplane you do not need any formal training at all for the 14 JAA exams. There is a form on the CAA website which needs to be submitted for their approval but you are exempt any training, you just apply for the exams as you are ready with the proviso that once you start they must be all be done in 18 months.
I purchased the Oxford Books and subscribed to the Bristol GS question database, I worked fairly hard (whilst working as a regional pilot!) and took a year to do the tests with 3 separate visits to Gatwick.
3. You get an MCC credit by submitting another CAA form based on your experience, your company may need to sign it.
4. You need a UK RT Licence, you can get one from US based rt examiners, I got mine in Florida.
5. You will need an LST for the issue of an ATPL. This has to be done in a JAA CRJ sim, I did mine with Lufthansa CST in Berlin, they will most likely be aware of what is needed. You will need to have the test observed by a UK CAA Inspector (at huge expense to you) again there is a form to apply for this.
As another fellow mentioned ALL of this may be different come September. I specifically got mine done before EASA because it could be different...
I'd get the medical and get approval to self certify for the exams which should grandfather you in for the writtens...
PM if you need more info...
Cheers