Essentially the CAA approve those FTOs for UK-issued JAR licences. Search and check "Standards Document 31" on the CAA website. Note the very few US businesses offering CPL and there are definately no full JAR IR courses out there.
LASORS in section iirc A8 describes the barrier.
www.caa.co.uk/lasors
If you can swing some residency time in Spain, you could get aournd the problem.