I suspect history is not your strong point, but mass migration in Britain started in earnest in the post-war years and without it the country would have never have fully recovered after World War 2. Migration was actively encouraged by all parties to re-build the economy.
You might also be unfamiliar with Britain's role in colonising half the world! As Britain decolonized in Africa, the Caribbean, and Asia, many people with British citizenship or ties to the UK migrated for better economic opportunities or to escape instability. Maybe some disapproved but I'm sure they hadn't disapproved of the colonisation and the Empire.
Now admittedly the next wave in 2004 after EU expansion might not have been approved by all; and ended up with Brexit. However, migration since Brexit has not declined much!
Anyway, I hope this helps.