Legally.
For civil disputes. Not criminal ones.
if 2 people have a dispute, they can agree to a 3rd party to sort if out for them.
jewish courts operate through this.. as do "sharia" courts.
their powers are limited.
there has been talk by muslims of extending those powers. But mostly in the press we get this nonsense -shock horror- articles, about sharia sharia law being adopted in britain. I'm sure muslim arbitrators going by islammic law have been arbitraring for decades.. it's not big deal.
It would be interesting if muslims push for capital punishment in their courts.
None of this is any danger to regular britons. You only face the judgement of whatever civil arbitrator, if you agree to go to them. If a sharia court gets more powerful, it would only affect muslims anyway, because regular people wouldn't go to a sharia court.
Of course, extending the powers of these courts would be pandering to muslims, making things more comfortable for them, and there's a danger in that.