all UK schools who did well were among the most advantaged in the UK in terms of social economics and cultural status.
Why is anyone surprised or even depressed by that?
Aspirational families support their children to do well at school. No matter how brilliant the schools, the children from the supportive families will always do better because they are learning at home too ... even if not academically, then socially and culturally. And by "aspirational" I don't mean "middle-class" - I mean aspirational. Almost by definition there are plenty of aspirational families in immigrant/migrant populations who won't stay at the bottom of the tree for more than a generation or so. They will be the new middle class eventually.
Cities tend to have more migrants and therefore more aspirational families than rural areas, and London is the biggest city in the UK. Of course it is going to top the league (so long as teachers can afford to live there).
We need to worry about areas that have entrenched poverty-of-aspiration, and we need to pour resources into bouncing them out of it, but we shouldn't be surprised or depressed if they don't top any league tables in the meantime.