The UK Arts is expected to burn at a temperature of 34C (93.2F) as the warmer weather reaches its fastest peak on Friday.
The Met Office said temperatures were expected to reach 34 degrees Celsius in London and possibly parts of eastern England, making it the hottest day of the year so far.
27-30C is expected in most parts of England and Wales, far from the capital and southeast.
Northolt in West London recorded a high of 29.5C (85.1F) on Thursday, surpassing the 2022 high of 28.2C (82.76F) recorded at Kew Gardens on Wednesday.
Cardiff recorded 26.1C (78.98F) on Thursday, 21.4C (70.52F) in Edinburgh and 20.6C (69.08F) in Derrylin, Northern Ireland on Thursday.
While England and Wales will experience heatwaves on Friday, Northern Ireland and Scotland will remain cold, both affected by the rains.
Meanwhile, firefighters are warning that the risk of fire due to heatwave has increased.
The London Fire Brigade (LFB) said it had already participated in more than 1,430 grass and open ground fires across the city this year.
These include meadows, canal and riverbank vegetation, shrubbery, parks, playgrounds, crops and forests.
About 350 of them were in home gardens, and the LFB said that with prolonged hot weather, their numbers are likely to increase.
In the last five years, London firefighters have been involved in nearly 600 fires, including barbecues, 45 of which have been set on private balconies.
Charlie Pugsley, the brigade’s assistant commissioner for fire safety, said: “We want people to enjoy the wonderful weather and do it safely.
“Barbecuing on dry grass is reckless and can easily cause a really serious fire – can immediately damage the area and endanger nearby property.
“We are urging people to think twice about having a barbecue on the balcony.
“Spreading balcony fire to others is much easier than you think, which can not only displace you but also displace hundreds of your neighbors.”
“We’re not trying to enjoy the heat wave, but for the sake of our city – and for our firefighters who have to work in high temperatures to deal with the flames – we really want people to Get our advice on board
“We are telling people to be vigilant and call 999 as soon as they see any signs of weeds.”
Firefighters are also warning residents not to risk their safety for cooling swimming.
In the last five years, staff have participated in approximately 300 reports of a person in or out of the immediate danger of entering the water.
The LFB said there were nine accidental drownings in London last year, making it joint third in the UK.
The UK Health Security Agency (UKHSA) and the Met Office have issued Category 3 Heat Health Alerts for London, East England and the Southeast.
The warning follows a Level 2 alert issued Tuesday and confirms that the Met Office will reach temperatures for the alert in three regions from Friday.
Level 2 alert is maintained for East Midlands and South West.
The Meteorological Department warned that difficult sleeping conditions could develop on Thursday night, with temperatures expected to be between moderate to high temperatures.
Predictors said that if the temperature does not fall below 20C (68F) anywhere, it will be called a tropical night.
The UK’s highest recorded temperature in May 1976 at Southampton May Flower Park was 35.6 degrees Celsius – and predictors do not expect it to be higher this week.