Even if they go on a deportation spree to rid themselves of offending migrants, the population of naturalized Muslims is already enough to ensure Brits would be a minority. Their birth rate is low anyway, just like most of Europe.
It is not just the high number of births. The Muslim population in England is increasing due to a wide range of reasons. A lot of people are converting to Islam, mostly in prisons and drug rehab centers. Also, a lot of non-Muslim women are marrying Muslim men and then converting to Islam.
Terrorists in a recent destabilization attempt here in the Philippines (this was during the ASEAN Summit) turned out to have married and converted Catholics. One woman has married a man who then acted as a guide in the island they attempted to set up a base in (it's away from the traditionally Muslim province so they're operating out of their turf). Later a ranking police official was arrested as she tried to extract the group. Turns out she met her husband while interrogating him for a different terrorism-related case years ago (how he got out of that one is inexplicable).
Gaddafi once said that their women's womb are their weapon in subduing the kafir. Guess we can add roosters and kittens into that arsenal...
This is happening all over the globe. For example in Nepal, Muslim men (especially migrants from the Indian state of Bihar) prey on vulnerable girls. They convert these women and then marry them. Once the marriage is done, using these women they recruit teenage girls, often 15-16 years of age. They promise jobs as housemaids and shop assistants in India. Once these girls reach India, they are sold to brothel owners. For the Muslims, infidel women are just commodities.