Dave
You seem to have confused a few things, hopefully I can help you unravel a few things.
1) If you do a FAA CFI and fly 700-1000 hours you will have about zero chance of gaining an instructing job in the UK. Reason: You don't have a JAA Flight Instructor licence. I will add, there are a few flight schools in the UK offering training towards FAA licences so it's not completely 100% useless.
2) Both the FAA and UK JAA licences are ICAO but neither are directly interchangable between each other. When being ICAO licences comes into effect is when you are dealing with smaller countries. Some Far East, African and Middle East countries will recognize FAA, JAA, Canadian or Australian/New Zealand licences and directly change them for their countries with little more than an Air Law exam.
3) It would be better to do both the JAA AND CPL licences at the same time. Both recognize 'flight time' just have different checkrides and exams.
4) There is no longer a CAA CPL course. All CAA courses lead to a JAA licence unfortunately the JAA hasn't managed to come up with a helicopter specific batch of questions yet so students are still being taught for the CAA exams!!!
To put you in the best possible position for your first position my recommendation would be to do both JAA and FAA Commercial licences, an FAA Instrument rating and a FAA CFI and CFII (Instrument Instructor). The Instrument Instructor would put you in an excellent position for work using a J1 visa in the US and help you gain the valuable experience/flight time required. Consideration for a JAA instructor licence before your return to the UK 'maybe'.
Just as an extra you are shooting too low on your possible hours during the J1 visa's 2 years. Personally I still had a couple of months remaining when I gained my FAA ATPL on a J1 visa and I know of a few others that attained the same.
That was 9 years ago that I left the UK and I still haven't 'managed' to get a job there. But many other HAI graduates have and are in the North Sea, a UK Flight School owner, UK Police, Irish Coast Guard or like me, still working overseas.
Oh, there are others that spent the money and no longer fly or whom never got a job in the US........ or anywhere else for that matter. Your licences are no guarantee of employment!!!
Hope that helps.........
P.S. You would still have to budget for a US Private checkride before you can continue your study's for an FAA Commercial