many of them were in the govt. You seem to be somewhat baffled by this basic point. BTW what does 'he said he would do it' mean here? Is it invoke article 50, then that isn't and was never a plan.
How could members of the Government defy Cameron and, as members of his Cabinet, start preparing for Brexit? He would simply sack them. Remember, he only condescendingly allowed them to campaign for Brexit, if they wished, and only then some time after the Remain campaign had been launched.
Cameron said he would prepare us for exit from the EU if he lost the vote. That is a fact; and he welched on it. He was the only one, repeat, the only one, who could have initiated a plan for Brexit whilst his Government was in power. Got it?