Actually Sharia courts exist in the UK; only their jurisdiction is limited and when they come to a conclusion that is incompabible with UK law, then UK law prevails.
This was said as an answer by the parliament to a petition to ban them. You can still find the reply, if you search for the key words sharia courts and UK parliament. I just don't want to link to it as linking to voting sites (even closed votings) could trigger a deletion of the post.
There are some newspaper articles about them, too, like this:
https://www.theguardian.com/law/2017/mar/01/inside-britains-sharia-councils-hardline-and-anti-women-or-a-dignified-way-to-divorce
It seems they are only used by Muslim people for mostly family affairs, but in a way it is actually a parallel court system.
These might exist in other European countries, too, and it is a legit question whether there should be a parallel justice system or not. I don't agree with the frenzy about every citizen being subjected to them, but in fact there were reports from a German school where Muslim school kids asked their non-Muslim schoolmates to adhere to Muslimic clothing rules and wanted girls and boys to sit separately.
https://www1.wdr.de/nachrichten/rheinland/scharia-schueler-neuss-100.html
(in German, WDR is a public German broadcaster, like a local BBC).