Rain and thunderstorms are set to sweep the South East in the coming days, while north-western areas will enjoy drier weather with sunny spells.
The Met Office said temperatures have dropped across the UK following sweltering weather earlier this week, and torrential downpours that could cause flooding are expected in some southern areas.
The Environment Agency has issued 17 flood alerts, where flooding is possible in London and surrounding areas, and Kent.
A yellow weather warning for rain covering much of England, apart from Devon and the North West, has also been issued by the Met Office.
On Friday, temperatures will range from highs of 17C (62.6F) in London and 18C (64.4) in Cardiff to 16C (60.8F) in Belfast and 14C (57.2) in Edinburgh.
The mercury is set to rise a couple of degrees everywhere on Saturday, remaining stable on Sunday.
Met Office spokesman Grahame Madge said Blackpool will see the highest temperatures, while cloud cover cools southern regions.
He said: “In the South East, there will be rain every day for the next few days, and some places could see up to 80mm (3.2in) on Friday.
“The heaviest rain will be in places from Hampshire up to Yorkshire, where we are quite likely to see intense rainfall which will clear away tonight.
“Away from the South East, it’s a relatively fine day.
“The warmest temperatures we are likely to see today are 18C (64.4F) around the Irish Sea coast – so places like Blackpool.
“Under the cloud of the South and East we are going to see temperatures getting to 15C (59F), so much cooler than the last few days.”
He added that Saturday has the “potential to be drier” in the South but the deluge is set to return on Sunday and last into next week.
The Environment Agency has urged drivers not to plough through flood water, warning that just 1ft (30cm) of flowing water is enough to float a vehicle.
Surrey Fire and Rescue Service has also alerted drivers to “lots of standing water” on the roads, while the Driver and Vehicle Standards Agency (DVLA) has issued advice on driving safely through summer showers.
The DVLA’s tips include allowing at least double the usual separation distance between your vehicle and the one in front, keeping speed down and using dipped headlights so that other drivers can see you.
The Met Office’s Mr Madge said the heaviest downpours could clear in time for football fans watching the Euros match between England and Scotland in Wembley or Trafalgar Square on Friday night.
“Most of the heavy rain in the South and East will have largely moved away by 8pm,” he said.
“We are expecting the heaviest bursts to be continuing until about 6pm, but there is still a risk of rain afterwards.”