The Met Office said the mercury hit 20C in Kinlochewe, north-west Scotland, beat Friday’s high of 17.5C recorded in the West Sussex village of Wiggonholt.
Elsewhere, temperatures reached highs of 15.3C in Castlederg, Northern Ireland; 17.3C in Bridgefoot, Cumbria; and 18C in Valley, Wales.
It was the first time the mercury had risen to highs of 20C since October, according to forecasters.
However, the temperatures are expected to drop on Sunday, with highs of 13C expected in Wales, 11C in Scotland and England and 10C in Northern Ireland.
Met Office meteorologist, Annie Shuttleworth, said: “It will be a much cooler day on Sunday. It’s still going to be sunny for a lot of people and the winds will be a bit lighter tomorrow as well, but we’ve got colder air over the UK, so that just means the temperatures won’t get as high.
“So maximum temperatures may be 9C to 11C, so it’ll be three or four degrees cooler for a lot of people, but we could see highs of 14C expected (for) sheltered western areas.
“There could be the odd spot of rain in the south west or the north east through to the afternoon and into the evening on Sunday, but it’s going to be very light. On the whole it will be largely dry with some decent sunny spells as well.”
Monday will also see slightly cooler and cloudier conditions and some showers possible in south-east England.
But dry weather should prevail everywhere else, with sunshine particularly in western areas.
Temperatures are predicted to pick up again into Tuesday and Wednesday, reaching the mid to high teens along with more sunshine and dry conditions.
Ms Shuttleworth said: “There will be a bit more cloud around on Monday. But as we head into Tuesday and Wednesday we are keeping with high pressure so things staying dry and temperatures will increase again up to the mid to high teens.
“There will certainly be some long-lived sunshine but it might not be wall-to-wall sunshine like on a day like today when there’s hardly a cloud in the sky.”
The pleasant conditions are due to the jet stream tracking well to the north of the UK, letting high pressure dominate from the east, the Met Office said.