Many..., many... years ago when they decided that their "Colonials" in the "British Empire" were entitled to freely immigrate into the "Motherland"!
Great Britain began circling the drain at that point in time!
1-—We needed that manpower.
2—Overall, immigration has been good or ok for the UK since then. Its the one group causing problems. Prior to the 90s it was black vs white racial tension. Which has healed itself. Other groups like Sikhs and Hindus have been great. HK Chinese. White groups like Greeks, Irish and others.