Multiculturalism can work. Where it falls down is in the attitude that the incomer or minority’s culture is sacrosanct. Multiculturalism only works when it is bi-directional. Incomers need to be welcomed and integrated, and they need to actively integrate within their new community, and the wider community.
When minorities want to retain every aspect of their culture-of-source, while living in a completely different culture, conflict is inevitable.
All migrants should be taught English, for example. It is unacceptable that schoolchildren should have to be interpreters for their parents. Very often the mothers have least access to learning English, so how can they interact with their children’s education and healthcare? How can they trust people they do not understand? Providing interpreters should only be a stopgap while all family members learn English.
There should not be any faith schools that are outwith the standard state provision. Schools can have a faith ethos and follow faith practices, but must teach the full UK curriculum according to UK standards, and including equality between the sexes, and learning about other religions in a neutral manner. Children can and should learn about their heritage in out-of-school settings - language schools, madrassas, synagogues, temples, churches etc.
Extremisim, of any religion or ideology, is the problem, not religion itself. Religion can be a good thing, a guide to living an ethical life. After all, every religion has a version of the Golden Rule: treat others the way you would wish to be treated. Ignorance leads to intolerance, and both lead to extremism.