GtE,
Not sure about that: while an FAA license is ICAO compliant, you might have a problem flying a G-reg microlight?
And if you had a N-reg ultralight, you would be restricted in the number of days you could have it in the UK!
Would suggest this needs further exploration via the BMAA?
(We have a microlight instructor with a Canadian SEP licence but a UK microlight instructor rating, and I can tell you that took some sorting out - pprune helped and a small donation to people for their assistance!)