Temperatures reached 34C on Sunday as the UK braces for three days of rain and yellow weather warnings.
The Met Office said Charlwood, in Surrey, recorded the highest temperature with 34.1C.
It comes as a search is also under way for a man in the River Thames in west London after he was reported getting into difficulty in the water.
The Metropolitan Police said a “multi-agency response” was in operation after they received reports of the man in the water at Hampton Court at 4.12pm.
A spokesman for Scotland Yard said: “Officers from the Met and Surrey Police, the National Police Air Service, as well as the London Fire Brigade, London Ambulance Service and RNLI, began searching the river.
“However despite the best efforts of everyone involved the man has not been found.
“Efforts to inform all of his next of kin are ongoing while work to recover the man continues.”
Heavy, thundery rain at first across Northern Ireland and Scotland this Monday morning ⛈️
Drier elsewhere, but thundery rain soon spreading from the west, feeling very warm and muggy for many 🌡️ pic.twitter.com/gVXOzUrduE
— Met Office (@metoffice) August 14, 2022
In Scotland and Northern Ireland, a yellow warning for thunderstorms was in place from 9am on Sunday as the north experiences a sweep of heavy rain for the next two days.
The severe weather began to impact stores as customers were evacuated from a Tesco supermarket in Inverness when water poured in through the ceiling.
Videos on social media showed tiles on the ceiling falling to the floor and water pouring in, covering most of the floor in the supermarket on Sunday.
From Monday, the warning extends to Wales and England.
The forecaster has warned of flash flooding, thunder, lightning and power cuts as showers beat down across the nations.
The warning remains in place in Scotland and Northern Ireland until 11.59pm on Monday and 11.59pm for all of England and Wales on Tuesday.
The south west and south east of England also face a third day of yellow warnings on Wednesday until 11.59pm, as the rain eases off elsewhere.
Dan Stroud, a meteorologist at the Met Office, said the drastic change in weather is due to an alteration in the air pressure.
He told the PA news agency: “We’ve had a number of days now where we’ve had clear, strong, clear skies and strong sunshine which has heated up the ground.
“We’ve had high pressure dominating, now we’re having low pressure dominate, so the air is becoming more unstable. As we’ve had some very high ground temperatures, it doesn’t actually take too much for the air to become even more unstable and for thundery showers to develop quickly.”
An official drought was declared in eight areas of England on Friday by the National Drought Group (NDG), which comprises representatives from the Government, water companies, the Environment Agency (EA) and others.
Three water companies – Welsh Water, Southern Water, and South East Water – have all imposed hosepipe bans, while Yorkshire Water has announced a ban will start on August 26 and Thames Water is planning one in the coming weeks.
Mr Stroud said that despite the forecast of intense showers over the next few days, it is unlikely to help the drought.
“It will help a little but to be honest really, it’s almost the wrong sort of rain,” he said. “What we’re likely to see is some heavy, intense downpours. With the ground baked so dry, it’s very difficult for the ground to actually absorb the water very quickly.
“So what tends to happen in these circumstances is the water runs off and we can potentially get some surface run-off issues, so some flash floods.”
On Saturday, residents in Surrey were without water after issues hit the Netley Mill Water Treatment Works.
Water supplier Thames Water apologised and handed out bottled water to residents in Guilford, Surrey Hills, Dorking and Horsham while engineers worked to restore the supply.
By Sunday morning the problem had been fixed, with residents being told water was “gradually returning to the area”. Just before 7pm, it said Netley Mill Water Treatment Works was running normally.
A statement from Thames Water said: “Netley Mill Water Treatment Works is now back in service and supply is gradually being restored to the local network. This will continue over the remainder of the day.
“We are very sorry that customers have been impacted especially at a time of high temperatures.
“When supplies do begin to return, we are asking customers to try to use this just for essential use initially. This will help us return supplies to everyone quicker.
“We are supplying bottled water to customers who we know need additional help. If anyone is unable to travel to a bottled water site they should contact us on 0800 316 9800 and we will provide assistance.”