You have to pass the appropriate theory exams and complete flying training sufficient to enable you to pass the CPL skills test. The costs of conversion can outweigh the savings made.
It is debateable whether your chances of getting a job in the UK are less having done much of your flying in the US. I suspect it depends on the employer.
As thecontroller asks in his usual pugnacious style, one factor to account for is that it is only very recently that the FI hours requirement has dropped from 300 to 250. Therefore, those who did their FI ratings a while would have paid for another 50 hours hour building.
However, I am interested as to whether your £45k for JAA CPL/FI included your accommodation costs, exam fees and licence issue (it all mounts up) and your FAA to JAA conversion costs.
Cheers
Whirls