The icy weather is predicted to continue for days to come, with more snow for eastern England overnight and on Friday.
Strong north-easterly winds are forecast for southern England, with warnings this could cause snow to drift.
More snow is forecast for central and southern parts of the UK on Sunday.
Many councils are restricting gritting to major roads as supplies dwindle, with supplies in some areas close to running out.
A survey of councils by the BBC found most had enough supplies to last another two to five days but were unable to meet guidelines brought in after the snowfall of February 2009 that they should have at least six days' supply.
Salt from Sweden
Many were hoping to replenish stocks soon under a government scheme announced earlier to help salt suppliers prioritise deliveries.
Transport minister Lord Adonis said of the wintry conditions: "They have been going on a lot longer than is normal, for even a severe winter and salt supplies are therefore under pressure."
However, he said local authorities and the government were working to make sure new supplies got to the places that needed them most.
WEATHER AND TRAVEL INFO
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Temperatures in some areas are already dropping, with the sea in Aberystwyth marina reported to have frozen.
The National Grid issued its second gas alert in three days, with demand expected to hit a new record of 454m cubic metres on Thursday.
The current record is 449m cubic metres, set on 7 January 2003.
A spokeswoman said technical problems had led to a supply shortfall of 52m cubic metres from the Langeled pipeline, which takes gas from Norway.
The National Grid said other suppliers were putting more gas into the network and major customers were cutting back on usage in response to the alert.
AT THE SCENE
BBC's Alison Harper in Clanfield, Hants
Under 12in of snow, there are 100 cars abandoned as you enter Clanfield, but many are more concerned about the power cuts.
The unexpected outage sent temperatures tumbling. Many here used traditional wood-burning stoves. Some cuts lasted an hour; some into Thursday.
A family clad in pink fleeces and ear-warmers said they used their time playing murder-in-the-dark, before heading for bed.
A couple enjoying a brisk walk said they'd made the most of the power cut with a romantic candlelit dinner.
And while local people may have had different ways of dealing with the dark and cold, they did agree on one thing: they expect more power cuts this cold snap, but next time they intend to be better prepared.
Ninety-five industrial users in north-west England, East Midlands and East Anglia on "interruptible contracts" were earlier told to stop using gas because of high demand from other customers - the first time this has happened in four years.
The number of firms told to turn off their gas will be reduced to 27 on Friday, National Grid says.
The firms have agreed to take lower supply levels when required by the National Grid in return for reduced tariffs.
Roger Salomone, of the Engineering Employers' Federation, said the move had exposed the "longstanding vulnerability in our energy system", showing the nation had to "take security of our energy supply more seriously", including increasing gas storage facilities.
About 5,000 homes across southern England were without electricity because of trees falling on power lines and safety mechanisms being triggered by ice weighing down cables.
EDF Energy said only about 400 to 500 of those homes remained without power by Thursday night.
Most of those cut off in Kent, Surrey and Hampshire had been reconnected and it was reporting only "normal fault levels" for the time of year.
A further 970 homes were without electricity in East Finchley, north London. EDF blamed a fault with an underground cable which happened at about 2200 GMT.
Up to 25,000 homes across the UK have been similarly affected since Wednesday.
Eurostar is running a limited service after the breakdown of a Brussels to London train in the Channel Tunnel.
Eurotunnel is allowing only one Eurostar train at a time in the tunnel, which means delays for travellers.
All major airports are operating but passengers have been warned to expect delays.
Budget airline Easyjet cancelled more than 100 flights on Thursday, mostly at Gatwick, where a total of 136 inbound and outbound flights were scrapped. British Airways has also cancelled some flights.
Body under lake
Rail services have been severely disrupted by the weather, with train cancellations by several operators.
Widespread school closures have continued, with about 950 shut in Wales, 700 in Scotland and thousands across England.
Teachers' leaders have voiced concerns that more heavy snowfall over the weekend could throw next week's GCSE and A level exams into chaos, with schools closed and candidates unable to get to those schools which are open.
The Federation of Small Businesses has criticised head teachers for closing schools too readily, saying lost working days are affecting many companies.
• Police are concerned for the safety of two teenage Welsh girls missing in sub-zero conditions near Newport. They have not been seen since Sunday
• An angler died near Newbiggin-by-the-Sea, Northumberland, after severe weather caused the cliff he was standing on to collapse
• A man's body was found under ice in a lake at the Lakeside Country Club in Frimley Green, Surrey, on Thursday morning
• A teenage boy died and his mother was seriously injured when they were hit by a lorry on the A1 in North Yorkshire on Wednesday night
• A 42-year-old man also died on the A1 when his car crashed with a lorry south of Cockburnspath in Berwickshire
• A man was treated in hospital after being injured when the weight of snow caused a cattle shed to collapse in Aberdeenshire
• Dozens of farm buildings are reported to have collapsed under the weight of snow in north-east Scotland
Overnight temperatures on Wednesday plunged to a low of -18C in Woodford in Greater Manchester and Benson in Oxfordshire. Monitors recorded -13C in Manchester, -6C in Glasgow and - 3C in central London.
Up to 5cm (2ins) of snow was forecast for eastern England during Thursday, with snow showers possible in Aberdeenshire, the Midlands, the South East, Pembrokeshire and Cornwall. Stronger winds could lead to some drifting.
Sporting fixtures continued to fall foul of the weather, with race meetings at Kempton and Lingfield called off and the weekend's racing, rugby and football fixtures under threat.
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