This is not what the BMAA seem to say. I was told unequivocally that it is not legal to fly microlights in the UK on an FAA certificate. As far as the FAA are concerned I would be legal, but in the eyes of the BMAA apparently not. It is unclear how this is reconciled with the ANO section on recognition of other ICAO licences.
If by UA you mean an ultralight, believe it or not you don't need any kind of licence to fly the flexwing type in the US (not that I ever recall seeing one). The three axis types can be flown on the relatively new FAA Sport Pilots Certificate, daytime VFR only within a certain radius of the home field, and with some other restrictions. However the Private Pilot Certificate trumps this (ie you can fly ultralights without the need for special training or an add on rating).