Do you think ISIS thinks that? ISIS aside, Islam globally has an ongoing persecution complex.
ISIS accuses Alawites and Shias of practicing taqiyya, actually. It claims that they're not real Muslims, that they're just pretending to be Muslims so they can corrupt 'real' Muslims.
It's a way for them to justify murdering other Muslims.
Who's to say what the ones in the UK would do if they could get away with it?
And I'm sure many Christians in the US would love a country based off of biblical principles, but just like Sharia courts extending to matters of criminal law, it's not going to happen.
'Sharia courts' in the UK operate in the exact same way that Jewish courts do. Like many tribunals, their jurisdiction is based purely on consent. They can only bind those who accept being bound by them.
Is it a surprise that people are worried about Sharia when you can't get people to give a straight answer about the punishment for apostasy?
The punishment is death. It's debated whether it's death for leaving Islam or death for leaving Islam and fighting against it.
There's also a view that one must be a lapsed believer to be a Muslim. Therefore, a person who's born Muslim and leaves the faith, won't be considered an apostate as they were never truly a believer. They were just born into it.
The point is moot, though. Sharia tribunals adjudicate minor disputes and deal with some matters of family law. Even in Muslim countries like Kuwait, it's typically only applied in family and personal status law. Even then, it only applies to Muslims.
They're not going to be deciding questions of criminal law and chopping off hands.