Snow brings travel chaos to roads, rail and airports.
Luton, Stansted, Cardiff, Bristol and Birmingham airports closed runways and train services were hit.
Schools in parts of England and south Wales were shut and drivers urged to travel only if necessary.
The snow began to fall in the early hours, with southern areas and the Midlands badly affected. Forecasters said about 7cm of snow was lying across the South Midlands, extending across the Home Counties. "We just haven't had this sort of thing in recent years," a MeteoGroup UK spokesman said.
The snow was due to spread north, petering out towards midday but turning icy later.
Thousands of airline passengers were affected.
Virgin Trains said it would run revised services. First Great Western, Midland Mainline and Southern Railways were also hit.
The runway at Gatwick Airport was closed, but has now reopened, the BBC said. Luton Airport was shut due to bad weather, Sky News said.
Disruption to London's Tube services caused suspensions and delays on several lines, Transport for London said.
All Birmingham schools have closed, as well as some in areas including Essex and East Anglia. More than 300 schools are also shut in Wales.