There has been a lot said in this forum about such issues, but I will state what I know about the subject.
First the IR is NOT a licence, it is a rating. FAA licences are valid in the UK and on a G reg aircraft for non commercial use, however if you have an FAA IR you cannot operate a G reg IFR. You will of course get ALL your FAA licence privaleges if the aircraft is N registered.
Conversion from FAA to JAA IR is from my understanding not a stright forward case and you will need a MINIMUM of 15 hours training in JAA land for it. Note MINIMUM.
Look at where you want to fly and for what end. If you want to fly comercially in the JAA lands then do JAA training, if it is just for pleasure, why not do an FAA PPL and have fun flying in the US and then convert back home if you wish to add IMC etc.
Have fun flying and all best wishes for your chosen route.