IMC rating is only ever valid in the UK.
The IMC rating as a means to enter IMC conditions is indeed only valid in the UK. But... There is a restriction on a plain vanilla UK-issued (JAA or National) PPL that you may not fly out of sight of the surface, UNLESS an IR or IMC rating is held.
So having an IMC removes a restriction on your plain vanilla PPL, and that means that that restriction is also no longer present when flying on your 61.75 FAA PPL.
(Are we having fun with license restrictions yet?)