The hot weather led to several wildfires erupting across England while Scotland had heavy thunderstorms.
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.
Die Metropolitaanse Polisie het gesê 'n "multi-agentskap reaksie" was in werking nadat hulle verslae ontvang het van die man in die water by Hampton Court om 16:12.
'n Woordvoerder van Scotland Yard gesê: “Beamptes van die Met- en Surrey-polisie, die Nasionale Polisie Lugdiens, asook die Londense Brandweer, Londen Ambulansdiens en RNLI, het die rivier begin deursoek.
“However despite the best efforts of everyone involved the man has not been found.
“Pogings om al sy naasbestaandes in te lig duur voort terwyl werk om die man te herstel voortduur.”
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.
Die suidweste en suidooste van Engeland staar ook 'n derde dag van geel waarskuwings op Woensdag tot 23:59 in die gesig, soos die reën elders afneem.
Dan Stroud, 'n weerkundige by die Met Office, said the drastic change in weather is due to an alteration in the air pressure.
Hy het aan die PA-nuusagentskap gesê: “We’ve had a number of days now where we’ve had clear, sterk, 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, watermaatskappye, the Environment Agency (EA) and others.
Three water companies – Welsh Water, Suidelike Water, en Suidoos Water – het almal slangverbod ingestel, terwyl Yorkshire Water aangekondig het dat 'n verbod op Augustus sal begin 26 en Thames Water beplan een in die komende weke.
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," hy het gesê. “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.”
Op Saterdag, 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.”