Multiculturalism, when done right, is amazing.
Multiculturalism, when done badly, can cause a lot of hassle.
Unfortunately, Britain's ****-poor attempt at multiculturalism has caused way too many people to live in small enclaves where people don't like others, an us-vs-them mentality, clinging to ancient doctrines and nationalities. Thankfully, not always, but way too many.
And unfortunately, when people live in closed-off us-vs-them societies, frustration and hatred between groups can build and grow.
When the failures of this form of multiculturalism are ignored, or called racist and dismissed -- as it merely angers and alienates a lot of people who've encountered problems, and it encourages many people who've noticed the problems to support racist groups, because they feel ignored. This has happened with Britain for quite a while.
... And then, the UK government did a complete 180 turn and effectively blamed the immigrants and their descendants, alienating them and also making some natives think that the immigrants and their descendants are responsible.
As for the video, it's not that Muslim extremists are taking over the UK, but they are becoming more and more vocal. They're a tiny number of people, but they're given a lot of leeway (more than a lot of other groups would be if they did this crap) and are made out to be the representation of Muslims; they aren't.
There's many Muslims who do not want to live under shari'a; I expect that out of a majority of Muslims who do support shari'a, more than half would actually hate living in a shari'a state if they did; the idea is more appealing than it would actually be.
Sometimes, though, I see a elements of current, mainstream British 'culture', and I'd rather live under a shari'a controlled government.
I can understand why some people would support it when they see the failures of modern Britain.
I don't know what we could do to undo the screw ups of previous British multiculturalism; I don't think we can... :sad: