Just to revive this old chestnut, after looking at the FARs, and assuming you have 35hrs PIC in an aeroplane this is the best, most economic and fastest way to end up with a JAR fATPL:-
Train directly for the FAA CPL/IR and convert, and this is what you can expect to pay:-
25 hrs Seneca- $5375
20 hours sim- $1800
CLP & IR Test $700
Written tests $180
Books etc $100
Plane for test $380
With written exams out of the way - three weeks in the USA.
This assumes 35 hours airplane PIC. Any shortfall on 35 hrs PIC can be made up in a C152 at $56/hr.
UK conversion £5000-£7000 approx - depends on where you want to do the conversion.
You are excempt from many of the formal FAA requirements by virtue of your helo hours.
Rgds
EA