UK has long pursued a strategy of "social mobility", which is shorthand for: some places will be shit, and if you're hardworking or clever you can and should leave.
So the bright, capable people from white working-class towns either joined the middle classes or skipped the country entirely a generation or two ago.
Leaving behind a bunch of people for whom no wage will tempt them to London to do the hard, menial work needed to keep the city running. So on that front we have no choice but to import.
The people left behind either have caring responsibilities that means they can't move, long term health problems and disabilities, or just lack the basic work ethic, motivation and so on to get on in life.
Ultimately who's going to mop the floors at City banks and so on? It won't be the bright, ambitious kids of second and third generation immigrant families, not if they can at all help it, nor the sons and daughters of white middle classes, whether that's metropolitan elites or the trades and services people.
(This, btw, is why immigrants are generally hard working: "people willing to relocate their lives halfway around the world to an often hostile culture where they'll never truly be at home" is a strong filter for people with drive and motivation. Those lacking it stay home, regardless of which host and guest culture we're talking about).