If you do the FAA PPL and IR in the USA, then come back to the UK you need a minimum of 15hrs flight training only.
You still need to pass the writtens, but the mandatory ground school is "now at the discretion of the school" (that's my recollection of the wording; I can probably find the reference if necessary) which basically means you don't need to attend if you know the stuff.
For some people, e.g. those who fail the JAA audiogram
initial limits, it's also worth doing the FAA Class
1 medical, in the USA or anywhere, and doing the FAA
CPL too. That then gets you past the JAA Class 1
initial medical and you enter the JAA medical stream at the
renewal limits, which due to many old pilots being half deaf are much more lax
But this route works only with ICAO CPL or ATPL holders.