Parts of the southeast have experienced as much as a month of precipitation in one day, the Met Office said.
Hampshire and Buckinghamshire have experienced heavy rain over the past 24 hours, while Bedfordshire has also experienced wet weather.
Particularly affected areas were Otterbourne, which recorded 67mm of rain from 11pm Thursday to 5pm Friday, and Dancers End, Drayton Parslow, Toddington and southern Farnborough, which recorded rainfall between 65mm and 57mm .
The Met Office said temperatures had dropped across the UK following sweltering weather earlier this week, and torrential downpours that could cause flooding are expected in parts of the south.
The Environment Agency has issued 39 flood alerts, where flooding is possible, in London and surrounding areas, Kent, parts of the Home Counties, the West Midlands and Wiltshire.
A yellow weather warning for rain covering much of England except Devon, Cornwall and the North West has also been issued by the Met Office.
On Saturday, temperatures will hover between 19 ° C (66.2 ° F) in London and Cardiff, 18 ° C (64.4 ° F) in Belfast and 17 ° C (62.6) in Edinburgh.
This is because the mercury is expected to rise a few degrees everywhere on Saturday and remain stable on Sunday.
Met Office spokesman Grahame Madge said: “In the southeast, there will be rain every day for the next few days, and some places could see up to 80mm (3.2 inches) Friday.
“The heaviest rains will occur in places from Hampshire to Yorkshire where we are very likely to see intense rainfall which will subside tonight.”
He added that Saturday had the “potential to be drier” in the south, but the flood is expected to return on Sunday and last until next week.
The Met Office predicts that Saturday will have sunshine across most of the UK with some showers in Scotland and southern England.
Sunday could be “disturbed” in the south, with episodes of heavy and stormy rain in England.
The Environment Agency urged drivers not to cross flood waters, warning that only 30cm of running water is enough to float a vehicle.
The Surrey Fire and Rescue Service has also alerted drivers to “a lot of standing water” on the roads, while the Driver and Vehicle Standards Agency (DVLA) has issued advice on safe driving under heavy conditions. summer showers.
Advice from the DVLA includes allowing at least double the usual separation distance between your vehicle and the one in front of you, reducing speed and using low beam headlights so other drivers can see you.