1. Not true - You can look through this list and see that most of them are not Muslim:
en.wikipedia.org
Also, the mayor of London, for example, while Muslim was born in Britain and lived his whole life there. He isn't an immigrant.
2. The math just doesn't work for what you're alleging. The birthrates and immigration numbers just aren't what they need to be for the United Kingdom, whose population is nearly 70,000,000 and currently 6% Muslim to reach 50% in the next few decades. You've bought into some hype without working it out on your own to check it.