Human nature is such that most people subconsciously apply a selective filter to focus on data that reaffirms their convictions, while downplaying conflicting data. That is where external factors like educators and the media come in. The "liberal" media's very conscious and very deliberate tendency to portray minorities, especially blacks, in positive roles is one example of trying to redress people's perceptions.
In the case of Muslims, the Western media does the opposite: it seeks out and focuses on the most extreme elements and portrays them as being representative of the whole group. European media portrays Muslim characters almost exclusively in criminal roles. For many Westerners, they don't have extensive day-to-day contact with Muslims, so their only "local" point of reference is the media.
In this case, Britain has -- I don't know -- how many millions of Muslims. How many members does this Sharia vigilante group contain? Is this a large scale problem or media hype? If they are breaking the law, it's a matter for the police.
That's a strawman. I have already pointed out that, in a population exceeding a billion individuals without any central command structure, there are innumerable, self-appointed "leaders". David Koresh didn't represent Christians, any more than Meir Kahane represented Jews. The black panthers don't "represent" the African-American community. We all accept that there are similar elements in the Muslim community also, and the point is the tendency of the media to generalize and stereotype when any Muslim steps out of line.
Another example: a Lebanese Muslim cleric in Australia said something stupid about scantily dressed women and sexual assaults. The media and politicians had a field day ranting against "Muslims". Later, a white, female, Christian ex-judge also said something similar, but no one made an issue of it.