//'Extrapolating the numbers' is a bit dodgy, though. Because a) it assumes that the next generation of muslims will produce offspring as prolifically as their parents (many of whom are relatively recent immigrants from cultures where this is the norm), and b) the implicit fear - that the UK will be "islamified" - assumes that the next generation of muslims will be equally as devout as their parents. //
These are thoughtful observations.
Re the first one, Kromo, this was the argument used in Powell's day: the birth rate of the new young immigrants will fall to "normal" levels in time. This has turned out not to be true. This is, in part, because of the prevalence of first cousin marriage where the girl or boy go back to to the "home country" to get a spouse - a process facilitated by Jack Straw's repeal of the Primary Purpose law. (Now 70% of Bradford Muslims are married to first cousins). So the process of assimilation is constantly deferred.
Let's take Kromo's second point - the "implicit fear" of Islamisation. I think the opposite is true: the new generation is more devout (perhaps more conscious of its Muslim identity is a better way to put it) than its parents. And I can offer a number of explanations one of the most significant of which is radicalisation of British mosques by the import of the importing of Wahabbi and Deobandi clerics funded by you know who.
So, having read your late post, I think your assumptions are false.