Currently, you can fly a
G-reg, worldwide, VFR, on any ICAO PPL.
No explicit validation is required; the validation by the State of Registry for a G-reg (the UK) is automatic.
That privilege is supposed to end under EASA but I don't know when. It might be April 2012, summer 2012, April 2014...?
Obviously you can fly an
N-reg worldwide VFR on an FAA PPL but you knew that
What you should do is collect a
JAR-FCL PPL while you are at it. About 5-6 schools in Florida, and one in California, can do that.
After April 2014, on
current EASA proposals, all pilots whose
operator is EU based will need EASA papers, on top of the State of Registry papers required under ICAO. This will make an FAA PPL not very useful, unless you can get an N-reg plane to fly in which case you will need it (as at present) but you will need an EASA PPL as well.
That is why you should pick a school in the USA which can deliver both PPLs after one training course.
(Obviously you will need the UK medical and the 7 UK PPL exams as well).