I would not be surprised to see the UK enact something like the Ottoman millet system, and grant semi-autonomy to its various ethnic and religious communities to run their own internal affairs. I don't think this would be a good move, but doesn't seem too unlikely at this stage.
The UK's three biggest ethnic minorities do have some autonomy in their territories - Scotland, Wales and Northern Ireland.
The idea of giving minorities that do not have a well defined territory is a complete non-started, and there is no demand for it.
There is some extent to which religious groups can (as anyone one can) use arbitration for some disputes (but that is ultimately subject to secular law) as some Jewish and Muslim groups do or have procedures to decide matters for purely religious purposes as Catholic and Anglican canon law tribunals do (e.g. annulments and internal decisions).
I have no idea why you think anything more is even remotely likely.
