The billionaires want open borders, because it creates an eternal supply of replacement people who can keep wages down.
The liberals want open border because it creates a permanent power base to draw from.
Then there are those who want white people eliminated from the face of the earth and be a historical memory.
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).
Then there are people who want to see the country destroyed.
Exactly and many of those billionaires are part of or working for the GOP establishment as well.