Not all your 5 exams expire at once -only those written more than 18 months ago (but you probably know that). So I'm guessing the first to go would be Air Law. Considering you're (roughly) halfway in terms of hours, you may find it cheaper to rewrite Air Law etc than to fly all the way to the States, getting your M1 visa, finding a JAA FTO that will take you on for only part of your PPL & finding accomodation.
Also with the exchange rate is in the 1.50's at the moment, a huge part of the saving is now lost.
Just my opinion, and I presume you're over the most expensive part of your PPL (circuit bashing) - so use the crisp winter air to your best advantage.