Letting large numbers of foreigners into your country appears to be a ratchet. It is much easier to let them in, than it is to get them out again. They want to come, but they don't want to leave, so force will be necessary. It is so expensive to deport someone -- thousands of pounds per person -- that the natives will not or cannot pay. People are polite, and will not want to say to someone's face "our country was and would be better without you". It is harder to say "we don't want Bangladeshis in Britain" if you are saying it to a Bangladeshi. It is effectively criticism of that person. No one wants to be (correctly) accused of racism.
Immigration begets more immigration. Once inside the country, those immigrants become a power centre. Politicians must take their views into account, or be beaten by other politicians who do. If only two-thirds of the population is white, as in the United States, a politician who intends to deport the non-white third cannot count on any of its support. He must win three-quarters of the white vote, which is virtually impossible.
But it is more than this. People are exhibiting racial Stockholm syndrome. They are positively extolling the virtues of having Bangladeshis and Somalis in their country. They talk about abstract benefits, and ignore the concrete disadvantages. Crime and IQ statistics are ignored. People say "diversity is strength" when it clearly isn't. An entire ideology has sprung up to justify the presence of foreigners: multiculturalism.
Multiculturalism is best understood as a piece of rhetoric. It is not intended to be a substantial or coherent idea. No one is in favour of a monoculture, of everyone being the same. But the country was not a monoculture before mass immigration. We already have a variety of cultures across the globe: that is what different countries are for. There is no need to bring them here.
Culture is a red herring: cultures are not monoliths. Culture is a vague term: you could not count the number of cultures in the UK. It is possible to be in favour of some parts of a cultures and not others. It is possible to be in favour of some practices and not others. Opposition to mass immigration is not just about cultures and practices, it is about IQ and crime, and the problems and degradation caused by mass immigration.