The FAA accepts ALL previous training, anywhere in ICAO, towards the logbook requirements for the PPL, IR, CPL, ME, ATPL, whatever. Some UK based FAA training outfits will say otherwise because they like to make money
This is true and is usually referred to as "training as required" in the schools I have trained with, however, you are required to meet certain requirements before you are allowed to undertake the checkride. This includes the usual various xcountry, night hours, etc (sorry dont have exact requirements with me). I believe as part of this that there is also a minimum number of hours you must have undertaken with an
FAA certified FI as part of the IR course.
Your other problem will be taking the checkride, you will either have to go to the US to take it or I have heard of exmainers coming over here for a couple of weeks at a time but believe they charge quite a bit for the test!
J.