You cannot do a JAR IR in USA, it has to be in a JAR state.
Spain is best as cheaper than UK.
If you want to go down the USA route. You will need to get a FAA PPL and then do a FAA IR.
Once back in a JAR state you can confirm it in line with that states minimum requirements. In the UK it is 15 hours training and the IR skill test.
This route is not always cheaper; Firstly you have to do a FAA PPL (If you have had your JAA PPL for more than 12months you can apply for a restricted FAA PPL) then there is about 55 hours for the FAA IR, plus the test and the test fee (not to mention the flights to the US and the initial check out at the flight school and accomodation). You will also have to purchase USA plates and maps.
You will find that when you come back to the UK the minimum of 15 hours is very tight, as you have to get used to flying in line with UK proceedures & standards. You may therefore need more hours. Don't forget another test fee etc
You need to work all the prices out if cost is your objective. You will be comparing UK only/Spain only/ USA then UK/Spain. I think that you will find that spain works out best.
Hope all that helps.