Wintry conditions will be brought on by a “bank of rain” in the early hours with snow on the way later on Thursday, Met Office meteorologist Clare Nasir said.
Freezing temperatures ranging from 0C in London to -10C in the valleys in Wales are predicted overnight, picking up to between 2-5C for most places in the UK in the daytime.
Yellow alerts for snow and ice likely to cause slippery surfaces and travel disruption are in place for Scotland until midnight on Wednesday. And for Northern Ireland the same warning is in place until 9am on Thursday morning.
From Thursday afternoon until Friday morning, a yellow warning for ice covers the north and the entire east and west coasts of England and Wales. Only a small portion of central southern areas are free from alerts from the Met Office.
The Environment Agency has also issued four flood warnings for the Isle of Sheppey, Kings Lynn, Peterborough, and by the River Derwent in Yorkshire.
Some 30 alerts, for areas where flooding is possible, are also scattered across England.
There were no high water warnings in place for Scotland, Wales, or Northern Ireland on Wednesday night.
Snowfall is expected in the northern half of Scotland on Thursday, with 1-2cm depth covering lowland areas, and up to 10cm expected to fall in areas higher than 300m above sea level.
Ms Nasir said: “Watch out for some freezing fog patches (and) the risk of ice first thing.
“However, through the morning, we will see some brighter skies for southern parts of England and Wales.”
Ambulance services have urged people to plan their journeys ahead of setting off and to wrap up warm to ease pressures on emergency services.
The risk of icy weather and wintry showers is set to persist through Friday, with cloudier and milder weather moving in from the north and patches of sun across the country.
A yellow weather warning for snow is in place for north-east Scotland from midnight on Thursday until midday on Friday, and an ice warning covers Wales and much of England with the exception of some central and southern areas until 11am on Friday.