In 2008 the Muslim population was estimated to be 2 million. There are two aspects of it. For the Europeans this number itself is too large. In Muslim circles usually such numbers are taken as under estimations. If recent history is any guide then it usually happens that Muslim view point gets corroborated. And it was. The population of Muslims in Britain was estimated to be 2.4 million in 2009. It is difficult to have have an increase of twenty percent in one year by natural reproduction and reversion. Such increases can happen only after mass migration. The obvious conclusion is that earlier numbers were an under estimation. The buck does not stop there. The 2010 figure says that Britain has, in deed, 2.87 million Muslims. Again this can not be explained by usual arguments. The only rational explanation is that earlier figures were both under estimations.
One problem of under reporting population is that you can not explain the growth in the subsequent years. Naturally the fear of population drowning come to the fore. Muslims have little to do with this state of affairs but the consequence will be growing concern about their population.