Firstly, you're not required to believe in anything.
You do if you want to get on, if you don't want to be marginalised, ostracised or even cancelled. The two most successful ethnic groups in this country, measured in academic achievement and prosperity, are British Chinese and British Indians, somewhere down the bottom of this scale you'll find Bangladeshis, white working class young men are not doing too well on this scale either. None of this has to do with ethnicity, but it has everything to do with culture and the ability, or otherwise, of various groups to identify the prevailing culture in this country, what to swerve and what to grab, and make it work for them.
I grew up in a poor working class family in Lower Broughton in Salford, I had instilled in me from an early age two things, get educated and get out, not just from Lower Broughton, but out of the working class all together and into the professions.
Two questions. What do you define as British culture or Britishness? And do you think any of this Britishness is eroding?
You're missing the point, what defines British culture, or Italian, French, German, or Greek culture doesn't matter so much, it changes over time, what matters for a harmonious society is whatever the prevailing culture of a country might be at any one time, it is a set of values that the vast majority of folk in that country recognise and feel an affinity with, what defines them in their hearts and minds as British or French or Greek, shared values that unite them.
Multiculturalism is the exact opposite of this, it states all cultures are equal, there is no prevailing culture and even if there were, to integrate is a betrayal of your own cultural identity. We all know the slurs hurled at black people for acting like white folk and there are many other such slurs used by various ethnic groups for those who dare to bend with the prevailing wind to get on.
Multiculturalism creates a self reinforcing ghetto mentality, us and them, this perpetuates ignorance that can turn to anger, fear of one group swamping another, all nicely stoked up by the manufactured outrage of right wing bad faith actors.
And don't believe the line from the left either who see everything through the prism of class, that it's all deliberate, a contrived artificial scarcity of resources for the poor, so that different ethnic groups fight each other over scraps, while the fat cats play divide and rule.
While it is undoubtedly true that class is the single biggest determinant of life chances, multiculturalism exacerbates this, hindering struggling ethnic groups from developing class solidarity outside their group, preventing them from recognising they have more in common with the poor white bloke down the road than they might think, except all too often nowadays the white bloke isn't there, and even if he were, everyone is too busy living in their self reinforcing cultural bubble.