One final point to consider, the exam is much easier after you have spent a bit of time in the usa, you will pick up or reinforce quite a lot of the exam sylabus whilst flying or talking to other pilots. It does help. If you are prepared to just study the questionbank for long enough to learn it you can of course pass the exam in the uk, however the oral exam part of the checkride for both cpl and ir will be hard, and in preparing for that you will in any case be able to breeze the writtens. You can expect a couple of hours of really quite tough questioning for both cpl and ir, with the added issue of being asked in depth questions about the examiners pet topics. The written is significantly easier by comparison I'd say and only needs to be done just before the checkride so if you can say you are ready for the oral the written is just two hours out of your day when you are ready.
Anyhow, good luck with it. If you go to farnborough you should be able to get your IR tsa fingerprints done there as well at the same time, although I seem to remember it was a particular day every week or something. Hope it all works out for you whichever way you go.