Sorry to say this has been done to death here
The UK ANO automatically validates any ICAO PPL (or CPL or ATPL) for a G-reg, VFR, worldwide.
This is set to end in April 2014 or 2015, I think, after which it will be limited to Annex 2 aircraft (basically Permit types etc).
Is an FAA CPL totally useless (over a PPL) on a G-reg? That's a good question. I vaguely recall that a "CPL" (any ICAO CPL) has extra privileges on oddball stuff like the max radius of charity flights...
Some notes on the EASA crap are
here.