Meanwhile, music fans could enjoy the hottest ever Glastonbury Festival in the coming week with the mercury possibly hitting 34C - which is 10C higher than usual, forecasters say.
The weather is expected to stay dry in the lead-up to the weekend, with a 20 per cent chance of "isolated showers" on Wednesday and Thursday.
A "wedge" of hot air from France could blow over to the Somerset site on Thursday, resulting in the sizzling temperature.
Glastonbury's infamous mud could make an appearance at the weekend though, when the weather is expected to be "changeable".
The 900-acre festival site, being headlined by Radiohead, the Foo Fighters and Ed Sheeran, will host a population of 200,000 during the event, which runs from Wednesday to Sunday.
"It is mainly going to be dry, looking at the whole event," a spokeswoman for the Met Office said.
"Wednesday, Thursday and Friday look dry with good spells of sunshine. People should take care in the heat, drink lots of water and take sun cream with them."
The Met Office spokeswoman said temperatures were expected to fall by the weekend, feeling "cooler and fresher" on Saturday and Sunday.
"We would describe the weather as changeable," she added. "We could see a mix of sunny spells and showers.
"There is a wedge of really hot air that is going to blow over France.
"That air could waft over the south of England."
If that happens, temperatures in eastern and southern England could see temperatures of 33 or 34C.
"I would advise those going to Glastonbury to pack for all weather - bring your Pac a Mac and sun lotion," the spokeswoman said. And wellies would be a good idea too.
This year, Glastonbury Festival will play host to Labour leader Jeremy Corbyn, who will introduce US rap duo Run The Jewels and appear at the LeftField tent for a talk.
Clive Lewis, MP for Norwich South, tweeted: "Chuffed to be speaking on Sat June 24 at Glastonbury 2017. Recharge your activism at the Leftfield tent with special gueststar @jeremycorbyn."
Extra security precautions will be taken at the festival in the wake of the terror attacks in London and Manchester.
Avon and Somerset Police have said there is "no intelligence" to suggest a specific threat to the festival.
Ticket-holders have been told to pack light, place luggage tags or ID on all their bags and expect searches from before entering.
"Like the rest of the UK, festival-goers should be alert but not alarmed," assistant chief constable Caroline Peters said.
And Thursday could in fact be the third warmest June date on record, reaching 34C, according to the Met Office.
June's record temperature is 35.6C, set on June 28, 1976, in Southampton.
The mercury peaked at 30C in Teddington, southwest London, while clear skies and strong sunshine were also enjoyed by the majority of Britons.
Temperatures are expected to continue rising into the low 30s into early next week.
Met Office forecaster Charles Powell said: 'We have seen the hottest day of the year so far.
"Sunday and Monday will see repeat performances of Saturday, with a lot of England and Wales seeing dry, sunny, warm, humid weather with high UV, high pollen.
"The temperature though is creeping up a little bit day on day, so we may see 33C on Monday, probably in similar sorts of areas - southeast England, Greater London.
"All throughout this period, we will have some pretty mild uncomfortable nights - a lot of places in the mid to high teens for night-time temperatures."
The building heat is forecast to keep temperatures in the south and south east some 10C above the usual average for this time of year.
"The sun is as powerful as it can be," said Mr Powell.
"I would say we are in the midst of a heatwave.
"You can't really deny the fact that most places are well above average in terms of temperatures, and it feels pretty warm."
Western Scotland, however, is experiencing its own weather front and saw only 13C on Saturday, compared with the highs of around 25C seen to the east.
Public Health England (PHE) has issued a heat health warning.
Dr Thomas Waite from PHE said: "For some people, such as older people, those with underlying health conditions and those with young children, the summer heat can bring real health risks.
"That's why we're urging everyone to keep an eye on those you know who may be at risk this summer.
"If you're able, ask if your friends, family or neighbours need any support."
Unusual levels of UV are also being recorded in the UK at the moment - with the strength of the UV in some spots being as high as that in Cyprus and Gibraltar.
This has prompted warnings that people should take extra care in the sun.
The heat was so intense in London yesterday that at least five Guardsmen were stretchered off after fainting in their sweltering uniforms during the Trooping the Colour ceremony at the Queen's birthday celebration.
Dressed in full uniform, including a bearskin, one soldier fell forward out of his formation as temperatures soared to 28C in Central London.
As the band marched towards his position, the guardsman collapsed to his knees before landing face down with his hands to his side.
