"...soon to be a majority among London voters..."

I hear people cite trends a lot. As a biologist I am aware that various species will produce similar trends, population spikes after a year of good rains, for example. The thing is those are always population spikes. They are never population plateaus. The year of good rains can be followed a time or three by more years of good rains, and populations can spike very high - but they always crash. They never plateau.

Trends only last until a catastrophe ends them, and reverses them with extreme prejudice. There are underlying reasons, very obdurate and enduring reasons, that populations have formed longlasting societies where they are. When another population is driven by a few good years to suddenly go there, those rains quit suddenly, and those novel populations crash. This is ubiquitous in nature.

There are never, ever population plateaus. There are only population spikes, where the sudden rise in population is followed by an equally sudden crash. Then the baseline conditions return, until the next rains come again.