I try to explain...
Getting an FAA PPL based on your JAA PPL is free (apart from the £39 fee to the CAA).
For the airlines you will need multi so might as well get it and add it for free to the FAA PPL (apart from £39 fee to the CAA)
So at this point you can fly P1 in the States or JAR land.
Why pay for an IMC???? pay a few grand extra and get the FAA IR, now in an N reg aircraft you can fly airways. At this point you can get the CAA IMC based on you FAA IR obviously you will need to pay the CAA licensing fee.
Now at this point in an N reg full IR privelages. In a G reg in the UK you can fly IFR but not in the airways and limited minima.
As for getting the FAA CPL for the sake of about £4k you might as well get it done and transfer the FAA multi and IR to it, at least now you have the oppertunity to fly N reg aircraft for living. It also prepares you for the JAR ATPL's.
Now with your shiny FAA CPL/ME/IR it is time to do the ATPL's. Once you have sat these you can then convert your FAA CPL and FAA IR. To convert the IR you need to do 15 hours JAA IR training and JAA IR check flight.
Once you have done that you have an fATPL. There is nothing to stop you studying the ATPL's right from the outset, and that will save you some time.
One thing to keep in mind is the minimum hours you need to be able to start various courses.