It isnt. Islam is crap at proseletysing.
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....