Well, the thing is, even if it does happen, it won't happen like people expect. Studies show that young muslims growing up in the west, whereas many retain strong ties to their religion, are much more liberal than their parents. Young muslims growing up in the UK don't want sharia law or laws banning alcohol, homosexuality, pork, etc. Even if they avoid those things themselves, they don't believe they should be illegal. Sure, there are exceptions, and extremists who are born in the UK... but there's also plenty of non-muslim nutters who go round killing and doing crime. Crime and counter-culture is more an economic issue or psychological issue than a religious issue.
If Muslims ever reach the point that they are the majority, the majority of them won't be fresh off the boat- they'll have been living in a relatively free society and have no desire to be repressed. Many of them will have been born British and have a very British outlook on the world.
