Not entirely.
I do the 1 only paper exam (on a computer) over here at flight safety in farnborough before i go,
Yep, that is correct (or somewhere else, not sure about FS).
I go to the likes of florida and i have to get a full FAA PPL then i move onto my FAA IR.
If you like. You don't need a "Full" FAA PPL if you don't want, but can add the IR to the FAA PPL based on your JAA PPL (or other ICAO PPL)
once i have the licence and i fly back to england to convert and be able to fly g-reg aircraft under IFR its only 15 hours conversion with no skills test at the end?
Nope. You must also complete and pass the JAA IR ground school and exams (£1000+ and lots of work). You then need to do a minimum of 15 hrs training (note: minimum) and fly the JAA IR skills test and pass. Depending on how you have been taught in the US makes a big difference to the "minimum".