I'm a religious person, can someone else arrest me please?

Some guy in the UK, the Arch Bishop of Canterbury in fact, made a controversial statement last week about incorporating Sharia law into British law. I don't think I understand the controversy exactly, but from the little I know of exact Sharia principles, I don't see how any of those principles could be contradictory to British law. That is of course if we're taking a non-literal interpretation and recognizing that the law must serve the times that it is in. Since I am so ignorant of law type issues period, I'll avoid talking about the specifics and jump into the general implications.

Secular law and religious law, could the two exist side by side? Why not? Is it any different from the way religious institutions operate now? Religious marriage ceremonies are recognized by the state, in many western countries, so what about other ceremonies and customs etc?

I'm opening up the discussion to all my law-minded friends.