The same mass immigration is also present throughout the West.
One ideological basis comes from people like Clement Attlee, the postWW2 Prime Minister. Attlee, and others of his like from the socialist anti-colonial movement purported to believe that if the races mixed together, there would be no more wars.
I doubt they actually believed this, and just used it as a justification to reduce Whiteness in Britain.
In this was embodied in the 1948 British Nationality Act, which put every resident of any colony as equal citizenship with the UK. (Theoretically, everyone in India and the UK colonies in Africa could have moved to the UK. The criteria was tightened in 1962).
Those who are attacking whiteness really just want to eliminate Christianity. For them, Christianity is the enemy and Western Christianity more so than anybody else.