Muslims don't have to proselytize. They start off as a very vocal minority demanding special rights (Sharia courts, burquas, special work regulations to accommodate their religion, etc, etc). Their demands are being met, their birth rate is much higher than the UK's birth rate (which is in decline). It's just a matter of time until Islam has as much political power in the UK as any other group....
Oh I agree, they grow because of a high birth rate, and although they are very bad at proselytising, its very difficult to convert Muslims, mostly because they impose severe penalties on “apostates”.
However, after years of decline, the UK birth rate is increasing again. And the muslim birth rate, although still higher, is falling markedly.