The scorching weather is expected to last into the weekend, with some councils forced to put in place emergency plans to tackle melting roads.
The UK enjoyed the hottest temperature of the year for a fourth day in a row on Thursday as the mercury soared to 33C.
Porthmadog in Wales reached the sweltering high, beating successive highs since Monday as the June heatwave continues.
The conditions caused some roads to become so hot they started to melt and stick to tyres, forcing councils to deploy gritters to spread crushed rock dust to improve traction.
Local authorities in Cumbria, Lancashire, Doncaster and Hampshire are among those using gritters this week.
The hot weather is expected to last with the sun set to shine into the weekend across most of the UK, as Sunday marks the first day of July - but parts of the country could be hit by thundery showers, forecasters have warned.
Helen Roberts, of the Met Office, said late on Thursday night: “There’s not an awful lot of change to be honest. Certainly tomorrow it’s going to be a repeat performance in terms of an awful lot of sunshine right the way across the UK.
“Another pretty hot day for most.”
She said an exception would be parts of the eastern coast, where low cloud will bring temperatures down.
Roberts added that parts of north-west England, north-east Wales, western Scotland and possibly Northern Ireland could hit around 30C.
The forecaster said there will only be “subtle” changes over the weekend, with very warm temperatures across most of the UK.
“We might start to get the risk of some thundery showers, particularly on Sunday, predominantly across the far south-west of the UK, so probably Cornwall is most likely to see any of that,” she added.
According to the latest information from the Met Office, Scotland’s top temperature was 31.9C in Glasgow, England’s was 30.6C in Keswick and Northern Ireland reached 30.5C in Derrylin.
The Met Office’s heat-health watch alert remains at level two, meaning social and healthcare services are at the ready to reduce harm from a potential heatwave.
Public Health England issued a warning saying the extreme heat may pose a risk to the most vulnerable.
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 found dead in the River Aire in Leeds in the early hours of Wednesday morning following a search and 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.
The RSPCA 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.
Commuters have endured delays after rail services were forced to impose speed limits due to overheating tracks.
South Western Railway said track 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.
Customers in some areas have been warned to take simple steps to help conserve water.
The Met Office said this is the first time in five years that temperatures have hit 30C in all four nations of the UK.