Big loss of face for Cameron and Ossie to Boris if that happened. I think the six months from Davies to the decision point is for the system to get the ducks in a row. That includes taking a view on all the risks---legal, protestor, cost escalation, the financing, the regulatory deal etc---and how to manage them.
If they go for it and it falls over subsequently that will be egg on face not some devilish plot.
Labour---aren't all four in favour?