The mercury rose to 31.7C in Coningsby, Lincolnshire, which is just half a degree shy of the top temperature of 32.2C recorded in Surrey on June 10.
The Met Office warned sun-lovers to enjoy the heat responsibly as a teenage girl died after being pulled from the sea and a grass fire was sparked in London.
The 15-year-old girl and a boy were airlifted to hospital off Cleethorpes beach in Lincolnshire on Saturday evening.
Humberside Police said: “We can confirm that a girl, aged 15, has sadly passed away following a multi-agency search to locate two missing children off the coast of Cleethorpes, North East Lincolnshire.”
The boy has since been discharged from hospital after receiving medical care.
Meanwhile, the London Fire Brigade said 10 fire engines and around 70 firefighters have been dispatched to tackle a grass blaze on Rammey Marsh in Enfield in the north of the city.
Around two hectares of grassland were alight on Sunday afternoon.
London’s New Wimbledon theatre cancelled a performance of Charlie And The Chocolate Factory the Musical due to sweltering temperatures.
It tweeted: “Due to outside temperatures in excess of 30C and its impact on working conditions on stage, we have reluctantly taken the decision to cancel today’s performance of Charlie and the Chocolate Factory the Musical.”
Met Office meteorologist Amy Bokota said: “Stay hydrated and make sure you are wearing sun protection as we have high UV levels.
“Make sure you drink lots of fluid and stay out of the sun.
“Make sure you are not putting any disposable barbecues on any grassland area.”
Thunderstorms and heavy showers are expected to hit northern England, Scotland and parts of Northern Ireland on Sunday evening, with up to 30mm of rain expected in some spots in the North East.
A yellow weather warning for thunderstorms is in place across Eastern Scotland and north east England from 1pm until 9pm on Sunday.
An identical one is in place across eastern parts of Northern Ireland between 2pm and 7pm on Sunday.
Temperatures over the weekend matched those of popular Mediterranean holiday hotspots, according to the Met Office.
Conditions are expected to cool next week with fresher air expected, particularly in the South East.
Ms Bokota said: “That really humid air is likely to feel a lot more comfortable than it has done for the last couple of weeks.
“Tomorrow will be fresher with some sunshine and showers like we have seen today.”
Temperatures in London are not expected to top 24C on Monday, representing “quite a significant drop”.
The blast of hot weather has come as Glastonbury revellers descended on Worthy Farm in Somerset to watch the likes of Sir Elton John and the Arctic Monkeys.
They can expect to stay dry during Sir Elton’s set closing the festival on the famous Pyramid stage on Sunday evening.
No “significant rain” is expected as they make their way home on Monday morning.