Rail failures and floods have wrecked travel plans on the busiest 24 hours of the Christmas getaway.
A signalling problem in west London meant the Heathrow Express was unable to run and services in and out of London Paddington station were severely delayed.
Train services near Gatwick airport, in West Sussex, were also affected by a major signalling problem caused by an overnight fire at Preston Park near Brighton.
Flooding also caused chaos as a number of rail routes were affected, with surface water causing several accidents and road closures.
Sky News weather presenter Nazaneen Ghaffar said more heavy downpours were expected, sweeping in from the east across much of the UK.
"About 25-35mm of water is expected - and falling on already saturated ground. So localised flooding is likely.
"Saturday will continue to be another unsettled day with rather heavy and persistent downpours spreading northeastward across all parts of the UK and Ireland."
The Heathrow Express was cancelled due to a signal failure at Hayes and Harlington in west London.
It meant that many of the 123,000 departing passengers were forced to take the Tube to the west London airport.
It also caused delays and cancellations to Paddington services, with those travelling to Cardiff, Reading, Bristol and Bedwyn in Wiltshire among those affected.
Flooding hit a number of train lines, leading to rail replacement buses between Liskeard and Looe in Cornwall as well as Chester and Wrexham General.
Services in Derby and Nottingham were also disrupted.
Motorists had to contend with floods in Reading, Bedford, Nottingham, Bromsgrove, Henley-on-Thames and Egginton, causing long queues on non-flooded roads.
Police also evacuated people from their homes in a Cornwall town as rising river levels threatened to flood their properties.
Residents were led to safety in Helston after the River Cober breached its banks.
Fire crews were called to around 70 flood-related incidents across Cornwall overnight.
Many of the estimated four million Britons travelling abroad for the holiday set off today, with destinations including the Canary Islands, Tunisia, Cuba and Mexico.
From today until January 3, a total of 700,000 will be leaving from Gatwick airport, 320,000 from Manchester, 300,000 from Stansted, 125,000 from Luton, 116,000 from Birmingham and 75,000 from Glasgow.
Many hundreds of thousands will also travel across the English Channel by ferry or through the Channel Tunnel, with 160,000 heading off on the Eurostar.
On the Underground, the train drivers' union Aslef is stopping work on Boxing Day.
The strike has already led to the postponement of the London football derby between Arsenal and West Ham United.
Apart from the Aslef Tube strike, the main train problem is likely to be engineering work on the West Coast line.
This will mean there will be no direct services from Euston station in London to Milton Keynes, Birmingham, Manchester, Liverpool or Glasgow on December 23 and 24.
Direct services resume on December 27 but with two of the four lines closed up to and including New Year's Eve.