What exactly is your problem with multiculturalism, OP? Multiculturalism is s good thing!
That's debatable. It means incomers retaining the culture of their place of origin and not necessarily adopting the culture of the UK. So we now have 'multiple cultures' (hence the name) rather than a general overarching one which most people share.
This can be benign, and mean that we all get to enjoy fireworks on Diwali or join in with the festivities for Chinese New Year. Other aspects are not so benign, communities which stick to their culture so rigidly they voluntarily segregate themselves and have nothing to do with the rest of the population. Honour based violence, forced marriage, FGM, caste based discrimination, Sharia or Beth Din courts which actively discriminate against women, oppressive attitudes towards women, subverting the political system by using imported organisations like biraderi or clans to manipulate results, pretty appalling attitudes towards women resulting in crimes like Rochdale, Rotherham, Oxford etc which went unpunished and were allowed to continue due to cultural sensitivities. I could go on.
Multiculturalism is generally considered to be a bit of a failed system which has led to poorer race relations and far more tensions.