I have a son in University, and from what I see of his contemporaries, they're much more comfortable with diversity, (culture, race, LGBTQ concerns among others.) - there was diversity when I was at uni 4 years ago. I was comfortable with it.
Here's a couple good questions:
How much diversity do we want?
Is it possible to have too much diversity, so that the negative aspects outweigh the positive?
You and I are already hard-wired to feel what we feel, and I can no more understand your feelings than you can mine - well first, I understand where you are coming from. I used to feel the way you do.
And second, my opinion is based on more than me being 'hard-wired' or 'feelz'... white-majority countries are the best countries in which to live. That is self-evident and an objective fact when we look at the countries that top the best quality of life list. There may be multiple reasons for white-majority countries being the best countries (none of which is 'white superiority', i.e. racial superiority).
And I want to keep it that way, the white majority, for the UK.
The simple fact is millions of brown and black people are emigrating to white-majority countries.
You don't get millions and millions of white people desperately trying to reach black-majority or brown-majority countries.
And so another question for you, an American citizen: which country would you want to emigrate to? Which country is better than the US?
If the future is a brown one - where's the democratic mandate for this? Which Western country has had a referendum where the majority of its electorate want a brown future?
Political parties which openly espouse this sentiment will be crushed come election time.