Not all Muslims have stated that. The Brits have many Muslims in national and local governments who are not stating that is their goal and that's what creates confusion among Brits, particularly among the pussified progressive Brits who want diversity.
When the radicals behave violently and particularly when they demand an independent country within the boundaries of the UK, it exposes the moderate Muslims and progressive non-Muslim Brits as liars and fools. And this damages the Muslim plan.
They'll fall in line when the time comes.