I think they are on shaky ground, legally speaking. At the time of giving the quadruple lock guarantee, Mr. Cameron had already been advised that it could be overturned in Court, so the suspicion must be that the assurance was given as a sop to the religious/right wing section of the tory party and the population as a whole.
It is my recollection from an admittedly imperfect memory that actually several parish CoE churches would have been quite happy to go ahead and offer marriage ceremonies to gay couples.
That marriage is a union between a man and a woman for the purposes of procreation is a fundamental precept of Christianity and hence the CoE and RC churches is well understood.
If my understanding is correct, and there are parishes, vicars, bishops etc within the CoE willing to offer services for gay marriages, they must have arrived at some accomodation in their own minds between their own interpretation of their religion and what the bible actually says, otherwise their heads would likely explode from the cognitive dissonance :)
So a further split in the CoE, with some moving in the Unitarian direction, the rest travelling to RC?
Of course if the CoE was to be disestablished as the state religion, then it would be quite easy for them to refuse. It is because they are the state religion that a legal challenge will likely succeed.
I would quite like to see a disestablished CoE, and for the UK to become a properly secular state - no state religion, no 26 bishops in the Lords, etc...