They may well have to do that to make this law anyway......apparently, I haven't had to confirm this, but the legal definition of marriage being between a man and a woman is in the articles of the Church of England and this is enshrined as an Act of Parliament, so the argument is that you change the Act of Parliament you must first disestablish the connection of that Act to the Articles of the Church of England...which would have significant impact on the overall establishment of the Church.
If this is a valid legal issue then there will be significant problems and far more resistance to this, mainly because of the wider legal and constitutional implications with a move toward the disestablishment of the Church rather than the actual issue of Gay Marriage.
Which is exactly why I say the whole Christian establishment should facilitate this process if they know what is good for them. Everyone has an interest in ensuring this all goes through smoothly. If they resist though I have no problems with the subsequent consequences for the church.