1) It's a private, independent, fee-paying school. That means that the school gets to decide what dress-code it enforces, and parents are free to send their children elsewhere (such as a free state-school).
2) We have Catholic, Anglican and Jewish schools, too. Most of them are oversubscribed, and state-subsidised. All of them force their students to study their religion.
3) When I was in (a state-run) school, I was forced to read from the bible, sing hymns, and listen to sermons pretty much every morning. Of course, I could have "opted out", and sat in silence in the library for 30 minutes (which, incidentally, was also the punishment for forgetting homework or being late for school). Things have largely changed for the better (i.e. more secular state schools), but it's clear that parents who want their children to have a religious education will have to plump for independent, fee-paying schools, such as the one in the OP. I know you young, angry, upper-middle class white kids like nice, neat black and white solutions, but in the real world, sometimes we have to compromise; allowing independent, faith-based schools is a pretty good compromise between allowing parents a choice of schools for their children and ensuring that no-one is discriminated against in state-school classrooms.
4) How the hell is this "Sharia Law"??? In what way is this a "law"? This is an independent, fee-paying school enforcing a dress code. It's not a freaking law, and any idiot who decides it's a "law" deserves to be called an idiot. So, does anyone think this is in any way Sharia "Law"???