The problem is that in the UK it is terribly expensive to buy or rent a house. As a consequence, one wage earner is not enough for a family to survive. Women have to work - it is not usually a matter of choice, but of necessity. That makes it very difficult to have big families. Muslims manage because they select an area of a city, an older district with large houses, and begin moving in. A large extended family will all live in one house. Property prices in such an area begin to fall. It becomes uncomfortable for whites to remain there. Thus, more muslims can afford to move in.