Maybe it's just too easy in London to put up a venue and fill it?Maybe it's just too easy for passers by to stumble into a milonga on their way home?
If you have to travel long distances, cross-county even, to get to a milonga, you're obviously dedicated and care about your tango. If you're setting up a milonga where there aren't many dancers in the locality then you obviously need to work quite hard and care enough to do it, and do it properly to get dancers to travel, and keep travelling back.
This isn't the case in London. It's afflicted by half-arsed organisers. It's plagued by hoards of prancers who don't care or want to care about tango, and it's easy enough for them to carry on just as they are.
So maybe the answer is to put up barriers to tango in London. And if you can be motivated to get over them, then the overall effect is higher quality milongas. Not very democratic, I know, but then neither is entry to the Royal Society.