Well like you I am planning on getting my FAA licences first, then converting them to JAA. I know people who have done this and it is no problem.
UKFT do not offer JAA licences nor are they authised by the CAA to offer them. You can see a list of all organisations in the USA that can offer JAA licences here
http://www.caa.co.uk/docs/33/SRG_FCL_APPROVEDFTOS.PDF
If you study the JAA PTIP course carefully, they dont have any JAA licences included just FAA, so you would be expected to get your JAA ones on your return.
Also to work in the USA (unless you have a greencard) you will need a J-1 visa, this is a study/work visa. UKFT are not authorised to issue these either according to the USA website here
http://exchanges.state.gov/jexchange...rd.list&cat=14
So I ruled UKFT of my list quickly.
There are about 6-7 schools in the USA that offer JAA licences, not inluding the IR this needs to be done in your JAA state.
If you only want FAA ratings then you are better going to a school that only offers FAA ratings (they tend to be cheaper), you must check whether they are authorised to issue the M-1 visa. If you want to gain flight instructor experience, then you need a J-1 visa, there are about 4 schools where you can do this, flyaviator.com, flightsafetyinternational.com, and others.
There are many good schools over there, I can only comment on Naples Air Center as thats where ive been to get my PPL, they offer the M-1 visa, so you wouldnt be able to work there.
Hope this helps and good luck.