Dave,
Just cause you attend a flight school, it by absoloutely no means guarantees you a job, it all depends on finshing at the right time, if they need an instructor great, if not, bummer.
As for getting CFI work in the U.S, maybe you will maybe you won't you just can't guarantee anything, some people leave with all the FAA qualifications and no experience and no money left.
Its also worth bearing in mind if you do the frozen JAA Atpl you only have 3 years to get you instrument rating other wise it is a wasted excercise, (A JAA instrument rating costs an extra 30,000 pounds)
Really think about it, with a clear head, its not for those who have to budget, do not under any circumstances think
"Its o.k I will be heading on to a well paid North Sea job soon enough"
Because it is very very unlikely.