because Sharia law is all embracing and is a conceptual law just as Common Law is - therefore it makes up the law system of several countries, as does Common Law (not least the US minus Louisiana, and a lot of Commonwealth countries.
I could go on and on - but it impacts on Commercial law, for example the notion of interest, the principles of negligence and liability. It impacts not only on family law, religious law, criminal law...........
In that it is not a law system that the UK would embrace. If someone chooses to live to its principle tenets (largely based on Islami belief), then fine, and the same is true with Bhuddism, Judaism and all the rest - but with a caveat.
Common Law is our (UNDERSCORED) system and that must override Sharia principles in our countries, when there is a dispute that needs judicial intervention.