The heatwave is forecast to continue as temperatures are predicted to soar into the weekend.
Temperatures on Wednesday hit 31.9C (89.4F) in Wales – making it the hottest day of the year so far.
Porthmadog in north-west Wales beat the 30.7C (87.3F) recorded in Rostherne in Cheshire on Tuesday.
The Met Office warned that temperatures were still rising, meaning the record could be broken within the next 24 hours.
“We may see some thunderstorms on Sunday or Monday but nothing’s certain.
“But we can expect these sorts of high temperatures to be reached certainly until Saturday.”
The rising temperatures have come with warnings to be careful near water and in the countryside, where fire crews have been tackling wildfires.
The heat has already claimed casualties – a 17-year-old boy was recovered from the River Aire in Leeds in the early hours of Wednesday morning following a search a rescue operation.
It is believed he got into difficulties while swimming on Tuesday evening.
Elsewhere the body of 13-year-old Ryan Evans who went missing on Monday after getting into difficulty in Westport Lake in Stoke-on-Trent was recovered from the water.
The body of a man was also recovered from a lake in Nutfield, Surrey, on Monday.
The RNLI has urged those heading to the seaside to seek out beaches with a lifeguard.
A number of homes were evacuated and pockets of fire continue to burn across an area of around six kilometres, Greater Manchester Fire and Rescue Service said.
The RSPCA also called on pet owners to keep a close eye on their animals in the heat after receiving 625 calls about animals in hot environments between June 11 and 24 – the majority about dogs in hot cars.
A spokeswoman said: “(We’re receiving) almost two calls every hour, despite the charity’s key advice being to call the police on 999 in the event of an emergency.”
Commuters have also endured delays after rail services were forced to impose speed limits due to overheating tracks.
Network Rail’s extreme weather action teams are being activated across the country to monitor “vulnerable locations”.
Network Rail said hot weather can cause the steel on rail lines to expand and in some cases buckle, causing travel disruption.
Slower trains exert lower forces on the track, reducing the likelihood of buckling.
South Western Railway said temperatures were expected to reach 47C on many parts of its route this week.
Services on the London Waterloo to New Malden line are being subjected to speed restrictions from 11am each day until Friday.
The Met Office’s heat-health watch alert is currently at level two, meaning social and healthcare services are at the ready to reduce harm from a potential heatwave.
Public Health England has issued a warning saying the extreme heat may pose a risk to the most vulnerable.
And while the mercury continues to rise in the UK, England’s World Cup team will also have to contend with the heat in Russia as they take on Belgium in the final group game.
The Met Office forecasts the temperature for Thursday’s game at the Kaliningrad stadium to be 22C (71.6F).