This is using the same flawed logic that has had people shouting about overpopulation since the 70's: assuming current* population growth rate will continue indefinitely
The difference is that this is a change in the demographics of a population, not the overall growth, which is limited by resources available. The growth of Islam isn't constrained by anything other than the cultural practices that lead to them having more children than the rest of the population and the tolerance of the country for immigration. These things could definitely shift to balance the scales but there's no guarantee that happens and there have absolutely been many times in history where a native population has been displaced by the growth of an immigrant group.
Nicholas, is it clear that at any point in the near future, let's say two generations (30-40 years) the Muslim population will not be a majority in the UK? I don't understand why you continue arguing.
* well, not even current. 15+ year old