Re: i for one, welcome... (actually I don't but that's by the by)
You aren't. Prime Ministers are always at the mercy of their more unruly members of their party. Governments can, and do lose votes. The party whip is not a guaranteed instrument and there's only so much patronage to go round to buy off the trouble makers.
Of course it's governments with thin majorities that are most at risk. John Major's experience of office was torrid with a constant threat of rebellions. James Callaghan had major issues with his party too and he was prevented in dealing with issues just as he would.
There is no tradition in the UK of prime ministers who have true presidential powers. They are always vulnerable and many have been brought down by their own side. There was one such, and her name was Margaret Thatcher. If it could happen to her, it can surely happen to anybody if they fail to command confidence. That is how the UK parliamentary system works.