Converting an FAA to JAA PPL if you have more than 100hrs time isn't a big deal. LST with an examiner, UK Air Law, HPL and RT if you want to add a professional licence.
FAA IR to JAA IR isn't too much of a big deal either. 15 hours conversion rather than 55 for issue? Flying an ILS is no different over there but you may have difficulty in finding an NDB to practice NPA's with.
However, be very aware that things are different on each side of the pond. Two very different styles, environments and attitudes, not to mention prices.