Water companies have asked customers to conserve supplies by not using hosepipes or water sprinklers - as the heatwave continues into the weekend.
The first hosepipe ban began in Northern Ireland at 6pm on Friday, and Severn Trent Water has asked people to be "careful with their water".
The Met Office said the highest temperature on Friday was 32.5C (90.5F) in Porthmadog in north-west Wales and highs of 30C-31C (86F-87.8F) are expected on Saturday.
This compares with average temperatures for the time of year of 17C (62.6F) in the north and 21C (69.8F) in the south, senior forecaster Marco Petagna said.
"There's a lot of fine weather still to come," he said.
"There will be low cloud to start the day, especially across central and eastern areas of England and Scotland and possibly in Wales.
"But that will soon burn away through the morning."
The recent hot spell has put a strain on utility companies, which have been pumping millions of litres into their systems.
NI Water chief executive Sara Venning said: "We have maximised our water production and need customers' help to reduce demand.
"We are asking customers to take heed of the hosepipe ban and stop non-essential water use - using hoses and sprinklers is causing demand to exceed the capacity to supply."
Severn Trent said: "We're set for another hot weekend, and, with demand for water really high, we're producing millions of extra litres.
"We're asking customers to be careful with their water and for now avoid using the garden sprinkler or hosepipe."
Meanwhile, gritters - a sight more usually associated with winter - have been deployed in some areas, spreading crushed rock dust on to melting roads to create a non-stick layer between the surface and vehicles.