I would go the FAA route. From where you stand, you could expect to obtain the FAA CPL / IR for $15000 (maybe less). I would go ahead and get your IR in the States, and get used to flying IFR, get your Private Multi, with instrument privileges (same check ride as you have IR), build hours, then your CPL, ME CPL (simple add on as you have private ME), then the CFI and CFII. Now you can start building hours as an instructor, and when you have 1500 hrs take the ATP check ride. Now convert to JAR, which will probably cost you 20,000 GBP (ATPL writtens, IR course), and you are in a VERY strong position to get a job.
There are several advantages with going the FAA route. The IR is a lot cheaper, and is as good as the JAR one. when you come to do the JAR IR, you will pass with minimum hrs due to your IFR experience. I read somewhere that they may be changing JAR so an ICAO IR holder no longer needs to complete the FULL JAR course as it is at the moment. If thats the case, your conversion costs will be even lower. A US instructor can expect to get $25-30 per hour, and you're right about the J1 visa.....
good luck