Try it the other way around, from FAA > JAA......
At least the FAA is still free of charge.
In the UK, not until EASA ruins it all in 2014 or so. Until then, you can fly any G-Reg with an FAA licence effectively with PPL privileges, and if you have an IR you can use it like an IMC rating. No paperwork, verification, charges, or anything.
One of the very few things that are easier FAA --> UK than the other way round.