The south east, so far this tear, is running at 62% of the long term average for rain. The ground was dry before this heatwave. Humidity is low, (for the UK), so the atmosphere above us is not holding much moisture.
Compare with July 2021, when London had 2 1-in-400 year events in the same month.
arrgghhh flooding next, we usually get some terrific storms after a hot spell