Labour would lose a lot of support in London, surely? I doubt that more than a relatively small minority of London homes are worth £1.5m+, but from what I can see there’s a big concentration of them in certain areas - like Starmer’s constituency!
Poorer inner London is already Labour (though pockets are probably vulnerable to the Muslim vote). To judge by the local council elections, though, the suburbs are more contestable. I have friends in better-off bits of north and south London who all tell me their areas would be hit badly by any ‘mansion’ tax, unless it was set higher than £2m.
There’ll be plenty of twitchy London Labour MPs now. And Khan will be nervous too, I expect.