What's wrong with Sharia law in UK, is that the British people don't want it. End of story.
If people want to use Sharia Courts to resolve purely civil matters, that's fine. It can run alongside ACAS, Beth Din, tossing a coin or asking your Gran to decide as a dispute resolution mechanism. That might even save the state money by not clogging up the courts with such matters.
But Sharia has no place in our Criminal Justice system, unless the British people decide it's what they want. And that is not likely to happen.