Matt, I sent you a PM... but I'd like to add to that here in hopes it might be of some use to others.
You can piggyback an FAA Pilot certificate onto a valid UK JAA license. The process has been discussed on here before, but basically, you get an FAA certificate based on your UK one (very simple process involving only paperwork, no tests etc... but paying the CAA for the privilege of having them release your info to the FAA who don't charge you anything), and as long as you're UK one is valid (with medical, class ratings etc), so is your FAA one.
I don't know what JAA qualifications you have, so if you can enlighten me/us it may help answer your question better. If you were going down the FAA Commercial route, you could get this issued as a standalone certificate and not need to do an FAA private too, and this would negate any reason to get your JAA stuff current again as it would be a pure FAA certificate.
So, either get your JAA stuff in order if it's lapsed, and apply for a "based on" FAA certificate (bearing in mind it's only valid as long as your UK one is), or forget about that nonsense completely and just get an FAA Commercial Multi-Engine with Instrument.
Then, any CFI/II (w/MEI) can instruct you in your N-reg and get you up to speed on the subtle differences involved with the FAA training standards, and ensure you meet the necessary requirements and get the relevant endorsements in order to take a check-ride with a DPE somewhere.
If you've already got the N-reg, forget about the CAA (and associated hassle and costs) completely and go FAA all the way.