Well, among other things, supposing that immigration did essentially stop then it might make the effective growth rate of the Muslim population even larger, if it held steady. Or it could be that we change immigration from EU countries with a more open (or even more open) policy from the Commonwealth, including Muslim countries like Pakistan. Or maybe, in a stunning volte-face, the UK parliament decides to open its borders to various refugees in unprecedented numbers, from Syria, Libya, etc.
It doesn't matter. The point is that if the way in which the UK population grows changes (and it's largely driven by immigration) then Muslim-specific projections must also be effected, either because relatively speaking the rate is different or because immigration actually does impact the Muslim population in some way. It doesn't matter which; the point is that you can't extrapolate from current data in any meaningful way.
You couldn't anyway, but especially so at the moment because of the prospect of imminent policy changes that will alter the demographics of the country.