So yes, you are basically saying this:
"By the law of averages, there' s a chance we'll probably have a Muslim prime minister one day, therefore we're losing control of the country."
Doesn't this strike you as a little tenuous?
Hitler managed it
Hitler was elected on a massive wave of popular support (which was dipping slightly when he became Chancellor to be fair but it was still there). I sincerely doubt a Muslim candidate could receive the same in contemporary Britain.
Plus remember that Hitler was elected at a time when democracy was extremely unpopular in Germany, so his anti-democratic measures actually commanded a considerable amount of public support at the time. Could you say the same for the modern UK?
All it needs is for there to be an increasing number of Muslim MPs, who vote in a leader,
It isn't just MPs who elect leaders. I'm not quite sure how it works in the tories, but in the Labour party there's an internal electoral college in which votes from MPs and rank-and-file members are taken into account.
Even despite that, your argument here essentially rests on the assumption that Muslims will become the largest voting bloc in a majority of constituencies (or within one of the two main parties). Which as I say is impossible.