Muslims (some of them) try to abide by it as part of their religion, but it's not law in the UK.
So they would get guidance from their Imam about matters of Sharia law. They still must abide by all the laws that everyone else does in this country.
It's no more law than the doctrine of Sikhism, Hinduism, Christianity or Judaism are. Though many principles of the main religions are similar and focus on respect for family, decency and honesty.