It is a few years since I read on that, so I have no sources ready. As far as I remember the courts decide on cases that are uninteresting outside their respective communities: religious divorce and so for people who are already legally divorced and want to religiously marry again. The courts cannot (bindingly) decide on custody for children. There is not much potential for controversy, which is probably why the courts are so little known. The sharia courts which exist in Britain (oh yes!) have become attractive for non-Muslims too: they work quickly. Under the arbitration act you can take your differences about the plumber's invoice there if the other party agrees to that too. And sharia, developed in a society of merchants, is well able to settle such controversies satisfactorily. (The Guardian had an article ages ago)

I am still of the opinion ALL these religious courts should be abolished.

Regarding sources, I note that it was in Katrin's quote:

Rowan Williams and sharia law | openDemocracy

He also points out that there is already provision in English law for Jewish and Christian communities to have some autonomy over the governance of their religious affairs, without thereby putting themselves outside the law.

From what I can see, Williams only referred to Orthodox Jewish practice, though. On that, here is a source from the time of the controversy:

BBC NEWS | UK | Religious courts already in use

...Jewish courts are in daily use in Britain, and have been for centuries.>

British Jews, particularly the orthodox, will frequently turn to their own religious courts, the Beth Din, to resolve civil disputes, covering issues as diverse as business and divorce.

...Both sides in a dispute must be Jewish, obviously, and must have agreed to have their case heard by the Beth Din. Once that has happened, its eventual decision is binding. English law states that any third party can be agreed by two sides to arbitrate in a dispute, and in this case the institutional third party is the Beth Din.

_{*Lunatic*, n.
One whose delusions are out of fashion.}