JAR-FCL 1.240(a)(6) -
A valid type rating contained in a licence issued by a non-JAA State may be transferred to a JAR-FCL licence, subject to the appropriate proficiency check, provided the applicant is in current flying practice and has not less than 500 hours flying experience as a pilot on that type, provided JAR-FCL 1.250, [1.251,] 1.255 or 1.260 as applicable, are met.
This is interpreted by the UK (LASORS Sect F9.1) as -
A type/class rating contained in a licence issued by a non-JAA State may be transferred to a UK issued pilot’s licence, subject to:-
i) 500 hours flying experience as pilot on type, operating as P1 or P2 appropriate to the rating required (PUT time may not be counted).
ii) Operational experience as pilot on type within the preceding 5 years.
iii) Pass a Proficiency Check on type.
iv) Have met the requirements of JAR-FCL 1.250/2.250 as applicable, including knowledge of aeroplane performance appropriate to that type (please refer to Section F4 - Knowledge of Aeroplane Performance).
Note: No take-offs and landings required in aircraft as already type rated.
Applicants who hold a valid type rating but do not meet the experience requirements above will be required to complete a course of training at an approved TRTO. The CAA may consider a reduction in the amount of training required to take account of
previous experience on type, subject to a recommendation in writing by the Head of Training. Applicants will be required to pass the written theoretical knowledge examination and a Skill Test.
My guess would be that, unless you meet the 500 hours on type requirement, it will end up more expensive to take the US route as you will have to complete some/most of the type-rating course again at a JAA-approved TRTO