Seeing how heavy the rain was it was obvious there was a risk of flash flooding.
As can be seen there are several areas of very heavy precipitation scattered around the South East, some in excess of 50mm p/hr.
There was no Met Office warning either last night (BBC forecast) or earlier this morning. A quick look at their site showed a yellow warning in force although not clear on detail as the text was missing (the mobile site had clearly not updated).
The desktop site did however have the information.
Yep. The warning was issued SEVEN minutes before it came into effect.