I think the Archbishop is wrong by suggesting that Sharia is incorporated into British law. There should be a clear separation of church and state.
However, I see no problem with consenting Muslims using Sharia courts for arbitration purposes in civil matters, saving the time and expense of using the civil courts, but in saying that, any judgements reached should have no legal foundation.
(95% of the 7,000 Sharia "judgements" in the UK last year were in cases of disputed divorce - hardly a call for stoning and beheading)
"Be Right and Sit Tight" - Jesse Livermore, trading legend...