I'm doing my FAA IR with Orlando Flight Training at Kisimmee in Florida. They are a now a UK owned (Cabair) outfit. I've spoken at some length with their CFI and I am paying $3995 (approx' £2500) for the single engine IR, inc' ground exams. I've got the study materials and if you've got an IMC rating, a bit of a brush up and getting your head around slightly different American terminology and procedures and bobs-your-aunty!
Cabair also offer the (twin) conversion in the UK for a JAR IR, which is actually 7 hours flying and three in the sim, apparently - but not cheap, i.e. over £2K, they aren't yet offering a single engined version which is probably to do with JAR in my opinion. In any event it's cheaper, easier and more appropriate to GA to take this route.
There's been no mention of taking the JAR exams, in fact exactly the opposite. BTW, I'm told that the reg's only changed last September.
However, you also need to get an FAA licence conversion in order to use your shiny new FAA IR, which entails filling in the application form for a (free!) licence conversion and posting or faxing it to the States along with copies of UK licence, ratings and medical certificates. You need to post or fax a release form to the CAA (form SRG1160) so that they will release information to the FAA to prove that you are who you say you are. The CAA charge £15 for this service - what little treasures (i.e. should be buried under six feet of earth).
(forms are available at
http://www.parkwestair.com/foreign.html)
Maybe some sanity at last?