The bitter conditions show no sign of relenting as the mercury is set to plunge to -13 next week with widespread snow expected.
The Met Office warned Tuesday's temperatures will plummet to the coldest yet this year, as winter grips the north of the UK.
The south certainly doesn't escape, with Brits warned to brace for -10 overnight on Sunday.
It comes as forecasters said snow could affect the South East of England on Sunday night and also Monday morning.
The Arctic blast, dubbed the Troll of Trondheim, has swept down through Scotland and into England, Northern Ireland and Wales, and led the UK Health Security Agency to declare a level three Cold Weather Action Alert.
Households will have to nudge the thermostat upwards this week, piling additional pressure on the UK's energy network.
Things are only going to get colder, though, forecasters warned.
For most, there will be daytime highs of 5C, but at night it's a totally different matter - and for many areas it will be at least 4C colder than the current temperatures.
The Met Office outlook reads: "Today, wintry showers affecting northern Scotland, Northern Ireland, Wales and northwest England, with snow settling on hills. Sunny spells elsewhere, though a few freezing fog patches may persist. Cold for all.
"Tonight, wintry showers continuing around exposed coasts, with some further accumulations on hills and the risk of icy surfaces. Clear spells elsewhere and a widespread frost. Some freezing fog in places.
"Tomorrow, cold with widespread frost and some freezing fog patches at first, slow to clear in places and quick to reform later. Wintry showers again for exposed coasts. Winds mostly light."
For early next week, it added: "Staying cold or very cold, with further wintry showers, mainly for coasts. Freezing fog is also likely in places, this slow to clear. Severe frosts likely by night."
The Met Office has issued a yellow snow and ice warning for London and the South East.
It is active from 9am on Sunday to 9am on Monday. Two to five centimetres of snow could fall quite widely, with up to 10cm in some places.
The Met Office told the Mirror that although it is cold this December, a temperature of minus 27.2°c was recorded in Altnaharra in the Scottish Highlands on December 30, 1995.
Met Office chief meteorologist, Steve Willington, said: “As an Arctic maritime airmass settles across the UK temperatures will fall with widespread overnight frosts, severe in places, and daytime temperatures only a few degrees above freezing.
'However, the cold air from the Arctic will also bring brighter conditions, with some dry, sunny spells, particularly away from the coast and where winds are light it could feel pleasant in the sunshine. Some patchy freezing fog is also likely.
“Yellow National Severe Weather Warnings for ice and snow have been issued for parts of Scotland, Northern Ireland, Wales and the east coast and South West England.
"Showers will turn more wintry with an increasing risk of snow as the week progresses, particularly in coastal areas or over higher ground. There will be widespread frosts with temperatures falling to as low as -10°C overnight in isolated spots by the end of the week."
