Temperatures fell as low as -14C (6.8F) making it the coldest night of winter, as new amber warnings are issued for heavy snow showers in Scotland later.
Tuesday evening was the coldest January night since 2019, the Met Office said.
Bitter conditions and significant snowfall has forced schools to remain closed in Scotland on Wednesday amid yellow warnings for ice and snow across all four UK nations.
Forecasters say Wednesday could see the coldest January night for 14 years.
By the end of Friday, more than 40cm of snow may be seen on high ground in north-west Scotland, as wintry weather continues, the Met Office said.
Snow, sleet and rain are expected to continue blanketing northern parts of the UK, though there will be some sunshine.
Heavy frosts and freezing conditions are likely across virtually the entire country, with experts warning of treacherous pavements and roads.
The low of -14C was seen in a provisional recording by the Met Office in Dalwhinnie in the Scottish Highlands.
The forecaster had previously said on Tuesday overnight temperatures in snow-covered parts of Scotland could fall as low as -15C, which would make it the coldest January night since 2010.
"It is still one of the coldest January nights in the last five years," Met Office forecaster Liam Eslick said.
"Although we didn't get the lowest temperature overnight, it's quite possible we could see that again tonight. So it is definitely still on the cards over the next day or so."
The lowest temperature so far this winter was -12.5C, in Altnaharra, in early December. In January 2010, -22.3C was recorded in the same Scottish Highlands hamlet.
Temperatures in most of Scotland are forecast to remain at freezing or lower throughout the day on Wednesday.
The UK Health Security Agency issued an amber cold weather alert for England this week, meaning the NHS is expected to come under extra pressure and elderly people may be more at risk.
Icy winds blowing in from the Arctic this week have seen temperatures fall 5C to 6C below the average for this time of year.
The Met Office has issued two new amber warnings for frequent heavy snow showers covering Shetland, Orkney and parts of northern Scotland, disrupting travel across the region from 15:00 GMT on Wednesday to 1800 on Thursday.
The Met Office has also issued three yellow weather warnings for Wednesday. They are:
A snow and ice warning for Northern Ireland and northern Scotland until 23:59 on Wednesday
An ice warning for southern Scotland until 12:00 on Wednesday
A snow and ice warning for northern England and northern Wales until 12:00 on Wednesday
A snow and ice warning for northern and eastern Wales, and the south-east of England from 22:00 on Wednesday until 11:00 on Thursday
The UK is braced for sub-zero temperatures until the weekend.
Among those disrupted by the cold weather are schoolchildren in Orkney and Shetland where all schools remain closed.
In Aberdeenshire dozens of schools are either closed or opening late, while more than 13,000 pupils have been given the day off in the Highlands as 138 schools and nurseries have been closed.
South of the border, seven schools have been closed in Lancashire and three in Cumbria due to adverse weather.
The cold spell has resulted in automatic £25 payments for households in receipt of certain benefits being triggered in more than 220 postcodes where temperatures are expected to be below zero for a full week.
The assistance with heating costs for eligible households also applies in some parts of Wales and northern and eastern England.
Alongside the snow and ice, a risk of flooding also persists for some parts. As of 05:00 on Wednesday, there were also 20 flood warnings - meaning flooding is likely - and 65 flood alerts - meaning flooding is possible - in place in England.
