It's the policy of the Tory party to reduce immigration, has been for years, and is popular policy that wins them votes. The Tories don't actually do it and among the politicians you're right: there is no serious rejection, for the reasons you've outlined.

But among the population, you seem to have an opinion based on TV, Twitter, urban areas, or maybe the youth. But to most of the country multiculturalism and mass immigration is controversial at best, and often very unpopular. In fact you can (and I do) often find critics of mass immigration among immigrants themselves, and their descendants.