I can only speak for what I researched for germany, but being a member of the oh so jointly regulated JAA
, it might apply for the UK too, that you only need to get an RT license (and the FAA PPL of course) to apply for validation.
The FAA license can be validated or transfered; the latter certainly requiring to comply with JAA currency regs.
Validation makes it cheaper, since under FAA regs you only need to get a biannual flight review (min 1 hour flight time). That is probably not enough if one decides to pick up flying again after a few years break, but certainly cheaper than 12 hours of flying. After that you just need another validation letter.