Northern Scotland escaped freezing temperatures on Sunday night but southern England continued to suffer a cold snap.
In a reversal of traditional weather patterns, temperatures reached 10.1C in Kinlochewe in the Scottish Highlands, while the mercury plunged to minus 9.5C in Santon Downham, Suffolk.
Heathrow Airport recorded its coldest night since December 2010 and coldest January night since 1987 as temperatures dropped to minus 8.4C.
Dozens of flights due to leave Heathrow Airport have been cancelled amid slippery and icy conditions on runways.
An analysis of live departures by the PA news agency counted 17 cancelled flights to destinations including Amsterdam, Barcelona and Miami.
A British Airways spokesman said: “Like other airlines, our schedule has been affected by the continued freezing fog weather conditions experienced across London.
“We’ve apologised to customers whose flights have been affected and are doing everything we can to get them on their way as quickly as possible.
“We advise customers to check the website for the latest flight information.”
A Heathrow spokesman said: “Poor visibility is forecast this morning at the airport and across the South East.
“While there may be minor changes to today’s schedule as a result of the weather, we want to reassure passengers that our colleagues are working in close collaboration with our airline and air traffic control partners to get them safely away on their journeys as quickly as possible.
“We encourage passengers to check with their airline for the latest information.”
The Met Office tweeted: “What a contrast this morning. Stepping out of the door across the North West of the UK and you can ditch the gloves… though ice scrapers at the ready in the South East.”
It said Monday is beginning cold and clear with some freezing fog in the East and South East.
It will be cloudy with some patchy light rain in the North and West.
Temperatures are not expected to rise above zero in London until midday and are unlikely to top 4C later in the day, while they could reach 10C by mid-afternoon in Northern Ireland.
Sunday saw mixed weather conditions across the UK as milder air moved into northern and western parts, while southern and eastern areas remained cold.
The mercury reached 10.9C at Achnagart in the Scottish Highlands on Sunday but it was also the wettest place in the country with 15mm of rain.
Temperatures dipped to minus 9.7C at Benson in Oxfordshire, while Weybourne in Norfolk was the sunniest place in the UK with 7.4 hours of clear skies.
On Sunday night Cambridgeshire Fire and Rescue Service warned drivers to take extra care as icy conditions are likely to remain “for a few days”.
Network Rail said passengers were delayed at London Charing Cross because ice on the conductor rail meant trains could not draw power, but they are now working normally.
London Fire Brigade told people to be careful when using an open fire to stay warm.
It tweeted: “Don’t risk using treated wood on fires. Not only can they produce toxic fumes, but they are also more likely to spit embers when burnt, which could set alight to nearby objects.”