The UK has suffered its coldest night of the winter so far with temperatures plummeting to -22.3C (-8.1F) in a village in Sutherland in the Highlands.
Overnight temperatures of -10C (14F) were widespread, leaving commuters again battling icy roads and pavements amid "stretched" road salt supplies.
Many schools remain shut, with rail and air travel again hit by delays. Fresh snow is falling in eastern England.
The weekend could be colder, as another week of Arctic conditions is forecast.
Up to 4,000 homes are without water after a main burst outside the Royal Berkshire Hospital in Reading. The hospital was largely unaffected.
Thousands of schools remain shut, with warnings that some exam candidates could have to wait five months to sit GCSE and A-level modules in England, Wales and Northern Ireland if weather prevents them taking them next week.
Exams watchdog Ofqual said in cases where candidates would not have a second chance to sit papers in the summer, applications could be made for "special consideration".
This involves pupils disadvantaged by circumstances being awarded up to an extra 5% of the maximum marks.
Milk deliveries have also been disrupted, with tankers struggling to reach dairy farms.
Some farmers have had to dump supplies as few have storage facilities to hold more than a day's stock.
Hardy swimmers take the plunge in icy water at the Serpentine in London
Easyjet has cancelled about 30 flights at airports including Gatwick, Liverpool, Belfast and Stansted, while Norwich Airport has been closed.
British Airways said passengers should check the status of their flight before leaving for the airport.
Overnight it had asked passengers on some flights arriving at Heathrow's Terminal 5 to leave without their luggage, with bags being sent on to them later.
Train companies operating revised timetables include East Coast, ScotRail, First Great Western, South West Trains, Southern and Southeastern.
The breakdown of a train travelling from Brussels to London in the Channel Tunnel on Thursday has affected Eurostar services.
Supplies of road grit are close to running out in some areas, and many councils are restricting gritting to major roads.
Thieves using a lorry with heavy lifting-gear have stolen a grit bin with two tonnes of salt intended for streets and footpaths from a road in Newton Mearns outside Glasgow.
The Local Government Association admitted reserves were "stretched", while the government is helping suppliers prioritise areas most in need.
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A three-vehicle smash was causing delays on the M9, near the Forth Road Bridge exit, and a lane of the anti-clockwise M25 had to be shut, south-west of London, after a crash.
Meanwhile, snow and ice forced the closure of a lane of the M58 near Kirkby in Merseyside and one in each direction on the M48 Severn Bridge.
Poor conditions have also closed the A4051 Bwlch Mountain Road, near Merthyr Tydfil, and the A542 Horsehoe Pass near Llangollen, Denbighshire.
Meanwhile, 27 firms - 12 in the East Midlands and 15 in the North West - are still using alternative fuels after National Grid told them to turn off their gas supply, with demand for energy soaring.
At one stage on Thursday, 95 major firms were affected.
Environment Secretary Hilary Benn insisted there were "plenty of supplies" of gas, with storage about 70% full.
He said firms affected were on special contracts - paying less but losing supplies when demand was high - and that there was no need for domestic customers to worry.
Of some 5,000 homes in southern England which were without power throughout Thursday, all but "a few hundred" were reconnected overnight, energy suppliers said.
School closures
Up to 25,000 homes across the UK have been affected by power cuts since Wednesday, mostly because of trees bringing down electricity lines or ice weighing down power cables and triggering safety cut-offs.
Temperatures barely rose above freezing on Thursday, remaining between -8C and 0C in most parts of the UK.
The UK overnight low was recorded at Altnaharra, Sutherland. England's lowest temperature was -17C (1.4F) in Woodford, Cheshire.
Sennybridge in Powys was the coldest place in Wales at -14.7 (5.5F), while Northern Ireland's low was -11.1 (12F) at Lough Fea, County Tyrone.
The lowest ever recorded temperature in the UK was -27.2C (-17F) in Braemar, Aberdeenshire, in 1895 and 1982, and in Altnaharra in 1995.
Temperatures are expected to stay around freezing throughout Friday, with the treacherous conditions causing many schools to remain shut. They include:
Almost 800 - about half of the total - in Wales and about 20 in Northern Ireland
All schools in the Borders, Highland and West Lothian council areas, along with most in Aberdeenshire and Moray
More than 630 in north-western England, at least 310 across Yorkshire and Lincolnshire, 400 in the north-east of England
Around 650 in the South West, nearly 2,000 in southern central England, 350 in Essex, 450 in Kent and 600 in East Anglia
The Department for Children, Schools and Families insists schools should stay open "wherever reasonably practicable" during severe weather.
A spokesman said the focus of government concern was solely on persistent, unauthorised absence.
It was up to head teachers to make "common-sense judgements" about whether to authorise a child's absence on the basis they could not get to school, he added.
Major disruption has already been caused to the sporting programme over the weekend.
Four matches in football's Championship have been called off, and only three fixtures survive in Leagues One and Two. Ten Scottish Cup games have fallen victim to the weather.
Horse racing and rugby union fixtures have also been hit.
Are you affected by the "big freeze"? Send us a picture or a video showing how cold the temperature is where you live. Are you a sports fan? Has the cold weather affected your local club?
Send your pictures and videos to yourpics@bbc.co.uk or text them to 61124. If you have a large file you can upload here.
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