I have been flying on a 61.75 certificate, while living in the US for several years and have raised the point with both the local FSDO and a DPE while presenting the paperwork on my IR check ride.
They were both unequivocal on their interpretation of 61.75 in that a foreign medical was not required to meet the validity requirements of a foreign licence.
OK, I don't have anything in writing to back that up, but I certainly won't be taking a 5000 mile round trip to my nearest UK authorised AME (in Florida) to keep the UK medical running.