Boris Johnson seems to be coming rather late to the the issue of surface water flooding. As his Mayoral term peters out he seems to be desperately reaching round for some sort of legacy. He has a reputation as something of a climate change sceptic, so that may explain why he has had so little interest in the subject. Authorities like Brent have developed policies for reducing flooding over some years. The effect of these can be seen in the use of permeable paving, tree planting, the use of sustainable urban drainage as a planning condition and the encouragement of soft planting.
People sometimes blame the use of controlled parking zones for people paving over gardens, but I am not really convinced. The expense of paving over a garden would take a while to recoup from the money saved on parking permits, and I suspect it has more to do with people wanting minimal maintanence for their gardens.