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> I wonder how much of this is related to people having paved over their lawns and gardens.

Whilst that may well contribute, the main cause here was just the sheer volume of water in such a short space of time. I was right under the thunderstorm, we had about 80mm of rainfall in a couple of hours. That’s the equivalent of two months of normal rainfall this time of year.

When you get it coming in that quickly the water is going to run off everything.

Yeah, same here, it was pretty impressive. Worth noting too that this is coming directly on the heels of a week+ long heatwave, so unpaved ground is going to be baked hard and won't be soaking up much. (During water shortages TPTB always comment that summer rain doesn't help much, winter rain is the thing.)
That won’t help, of course, but that doesn’t change the fact these really are extreme weather events. They’d be causing damage and disruption anyway. I live in south east London. Last week my wife was driving home in a rain burst, she got stuck in traffic and took video of water gushing up out of a drain like a fountain. The road was near the bottom of a valley, and the pressure in the drain pipes running down the hill must have been enormous.
Do you have any data suggesting there has been rapid paving of lawns and gardens since last year?
There's no reason why it would be caused merely by paving over of lawns and gardens in the last year - there's a lot of year-to-year variation in weather like this, and without sufficient advance planning and restrictions to enforce this, it's really easy to build in ways that get away without flooding for a few years or even decades until they don't.
In Southern California we’ve paved almost everything from the Ocean until 50 or 60 miles inland.
Pavement definitely soaks up less water than a lawn but note that the soil in London is rich in clay so it doesn’t drain quickly. I would expect there are plenty of waterlogged fields at the moment, though runoff from paved land surely doesn’t help.