Well I, and my spouse fit the criteria set forth by this thread. It is legal, we even have an amended Marriage Certificate in the State of NC…
It unfortunately doesn’t benefit us much as employers have flat out refused to accept it for health-care, etc.
As far as the legality of it, only the parties that enter into the contract can annul it.
While this is technically true, the form that the doctor submits must explain why no surgical treatment has been carried out. According to some British trans activists (I’m thinking particularly of Stephen Whittle here although I believe I’ve heard it elsewhere too) the way this has operated in practice is that any explanation other than “impossibility” for one reason or another will result in a denial of a gender recognition certificate. Thus, you effectively do have to have surgical treatment, unless for some reason you can’t.