If you want a job in the UK, get a UK license. By the time you have faffed around converting an FAA ATP to a UK/JAA ATPL you will probably have spent as much as you would have doing the whole thing here and wasted several months if not longer.
UK Airlines generally prefer candidates who have gone the UK route and preferably CAP509 for ab-initio.You may think it unfair but thats the way it is.