There does seem to be two uses of the term "multi-cuturalism". Most people here see it as appreciating that there are lots of different systems, and that they usually have some merit.
This leads to enrichment of our culture, by taking up the interesting ideas.
However there is the one used by activists and politicians which is that people "should" be members of "recognised communities", else they will get shafted.
Thus if you belong to a big gang, you can get away with any number of wrongdoings, with the defence of "it's our culture".
As someone of Irish ancestry, raised in Britain, I must say I find the notion of "Britishness" both silly and bad.
Britain is a federation of sorts, being several indigenous nationalities who manage to mostly live in peace. It is common for white British people to regard Indian cooking as "British", and express the desire for it when spending long stretches abroad. A high % of the rest of British cooking is French, the language is a mutant cross between German, French and Latin, with bits of Hindi chucked in.
"Britain" is actually a relatively new country, younger than (say) the USA.
In as much as there is a "Britishness", it is not caring about differences. This is very different to respecting them. The multiculturalists go one about how great other cultures might be, and rather dishonestly "forget" to talk of their defects.
They believe (wrongly) that knowledge of different cultures makes people live in peace. This is utter rubbish. People live in peace when they don't care about what others do.
In Iraq two factions of Islam murder each other, even though they know each other rather well. My Israeli and Arab friends demonstrate understanding of each other's culture far superior to mine. Not a lot of peace there, and of course the various European and American oppressions of Jews were not based upon ignorance at all.
The Christian terorism in Ireland was between two groups who knew each other's culture well. Indeed pretty much the only people who understand the collective insanity of that province are from it.
Religious schools do seem to be on the increase and that's bad. Not because they fail to teach about other cultures, since most actually do this better than those without imaginary friends, but because of the mixing that leads to indifference.
What we need is kids to see other kids, and not care that their culture is good or bad. We need them to hate each other in the random and less destructive mode of supporting different football teams. Kids will hate each other, form gangs etc, whatever you do, the trick is to make sure that these are weak and small.
Religious or ethnic gangs can be big enough to threaten civil order, so it is simply stupid for the state to encourage them.
We don't need stronger communities, we need weaker ones. Look at the last dozen atrocities you've seen on the news. Typically they were "inter-communal". Football fans are thuggish louts, but the scale makes them relatively harmless.