The previous record of 14.8 degrees in Colwyn Bay, Wales in 2011 was toppled at 11am on Friday in Ryehill, East Yorkshire.
The following hour a high of 15.3C was recorded in Coningsby, Lincolnshire.
Edinburgh will see temperatures around 11 degrees but will only drop to about 10 degrees overnight.
Glasgow saw temperatures of around 12 degrees and 11 overnight.
Further north temperatures will dip a bit more overnight but most places should comfortably reach double figures throughout the weekend.
Areas of southern and eastern England including the West Country, London and Lincolnshire, saw the mildest weather.
Met Office spokesperson Stephen Dixon said the first reading had been taken at 11am.
“Our station at Ryehill, a small village in East Yorkshire has recorded 14.9 degrees today, which tops the previous record of 14.8. It has provisionally broken the New Year’s Eve record,” he said.
“We would expect that to climb further and reach temperatures we saw yesterday of around 15.5 degrees.”
The Met Office said there had been just 26.6 hours of sunshine over 30 days – 38% less than the national average for this time of year.
The figure places this month in the running to become one of the 10 dreariest Decembers on record, forecasters said.
Britain’s dullest December was in 1956, when 19.5 hours of sunlight was the national average, while the brightest was in 2001 with 64 hours recorded on average across the UK.