The Midlands, south-west England and south Wales have seen the highest temperatures – of at least 32C.
Thousands of fans travelling to a Stereophonics concert in Cardiff faced cancellations after heat damage closed a railway line.
One fan catching a train from Aberdare compared the scenes unfolding at the station on Saturday morning to “a nightmare”.
“It was steaming in there – it was so hot,” she told BBC Radio Wales.
National Rail has warned commuters of possible disruption to travel this weekend as overhead power lines and rails across Britain could be affected by the heat.
Meanwhile, a series of running events ranging from 10K to an ultra marathon in Eryri – also known as Snowdonia – were called off amid the “extreme heat”.
Temperatures hit at least 31C in Wimbledon, in south-west London, as Iga Swiatek beat Amanda Anisimova in the women’s singles final. The current record of 31.2C occurred during the 1976 women’s final.
However, the widespread nature of this heatwave means Scotland and Northern Ireland have also seen temperatures of 30C or higher.
In Perth, fire crews spent a second day tackling a wildfire, where flames could be seen creeping up a hillside.