Nobody chose to ""multiculturalise" either with or without consensus. There has just been a natural progression towards multiculturalism in society.
When a number of communities with different cultures live together in close proximity, there is a natural human tendency to set aside cultural differences and live together in harmony (with the exception of a few extremist or isolationist elements). This does not result from any high minded moral stance, but simply from the fact that most people want to live a peaceful and happy life regardless of their culture or ethnicity.
The only real alternatives to multiculturalism, such the Apartheid system which was tried and failed in South Africa, and "racial purity" programmes, such as those tried in Nazi Germany and more recently in Serbia under Slobodan Milosovich, are at best morally unacceptable and at worst crimes against humanity.
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Edit
Since another answerer has addressed my points with specific reference to Africans and people of the Islamic faith, I would just like to point out that I didn't mention any specific cultures or faiths above.
There are a multitude of people with all sorts of cultures, faiths and ethnic backgrounds in the UK. In addition to Africans and Muslims there are Afro-Caribbeans, Indian Hindus and Sikhs, Jews, Poles, Italians, Chinese and Japanese to name but a few. Not to mention Scots, Irish and Welsh who also have their own distinct cultures which in many respects pre-date that of England.
Those who have specifically mentioned Africans and Islam seem to be complaining about immigration, not multiculturalism per-se. Also singling out people of these specific cultures does seem to indicate an element of racism. If it wasn't for that I wouldn't have mentioned the word.