A man died in floods in Chew Stoke near Bath on Thursday as downpours led to evacuations and road closures.
The Environment Agency has issued 83 flood warnings for England and Wales.
BBC forecasters predict prolonged spells of rainfall and gale force winds will batter many parts of the UK overnight on Saturday into Sunday.
However, there is uncertainty about where the “most significant” weather will hit.
A Downing Street spokeswoman estimated that 300 properties in England and Wales had been flooded in the past 24 hours.
She added the Environment Agency was closely monitoring conditions and had deployed teams to try and reduce the impact. Environment Secretary Owen Paterson is due to visit flood-hit areas in the West Midlands later.
Storm force winds
Avon Fire and Rescue service said the dead man became trapped in his car under a bridge in Chew Stoke, which is in the Bath and North East Somerset district.
Specialist rescue crews could not reach him in time, and he was pronounced dead before he arrived at hospital.
Local residents believe the driver was trying to cross the swollen ford when his vehicle was caught in the current and swept downstream.
Weather warnings have been issued for the weekend, with the Met Office saying winds could damage buildings and uproot trees. More flooding is possible, especially on ground which is already saturated by Thursday’s rain.
Gusts of 60-70mph could hit south-eastern coastal counties of England and there is the potential for storm force winds over the English Channel, forecasters say.
In other flood-related developments:
Fire crews have rescued a man who became trapped in his car in rising waters on the A45 at Stone, Staffordshire.
Ten elderly people were evacuated from their homes in Exeter after a 20ft (6m) wall collapsed at about 20:20 GMT on Thursday. Hundreds of tones of rubble was dislodged and no-one was hurt.
A landslip at Upper Camden Place in Bath forced the overnight evacuation of the houses below. It was caused by a 7ft (2m) boulder becoming dislodged from a supporting wall.
A fallen tree is causing delays and cancellations to train services in north Worcestershire, London Midland have said. The tree was blocking the line between Blakedown and Kidderminster and has now been moved, but services are still disrupted.
In Ulverston, Cumbria, a hospice was evacuated after a nearby stream burst its banks, flooding bedrooms and offices.
Emergency rest centres were opened at Llandudno Junction, Caernarfon and Bangor overnight after drivers on the A55 in north-west Wales were forced to abandon their cars. The dual carriageway near Bangor was blocked for several hours. Single lanes were re-opened in the early hours of Friday morning with a 40 mph speed restriction.
Winds gusting at about 90 mph battered south-west Wales. In one major incident, the emergency services including an RAF Sea King helicopter rescued a driver trapped in her car in deep, fast-flowing water in a swollen river at St Clears, Carmarthenshire.
Whitesands in Dumfries flooded for the second time in seven days, and about 500 homes in the region lost power after a lightning strike.
A man died after his car became wedged under a bridge in Chew Stoke, Somerset
On Friday morning, the Environment Agency still had in place 83 flood warnings, indicating that flooding is expected. Most of them were in the south-west of England and the Midlands.
More than 150 flood alerts are also in place, meaning the public should be prepared for flooding.
The Scottish Environment Protection Agency has no flood warnings in place, but several flood alerts are in force.
Train services have been badly disrupted in parts of England, Wales and Scotland.
Between Devon and Somerset, 12 separate areas of rail track were impassable, and the line was likely to be closed for the next two days, said a First Great Western spokesman.
In Gloucestershire and Wiltshire, trains between Bristol Parkway and Swindon were being delayed by up to 60 minutes due to the flooding.
Train services between Bangor and Holyhead are suspended due to flooding on the tracks near Bodorgan. Arriva Trains Wales and Virgin trains are continuing to terminate trains at Gaerwen.
Flooding also caused delays to rail services between Lancaster and Carlisle and between Carlisle and Kilmarnock in Scotland. Buses replaced trains between Kilmarnock and Dumfries.
BBC weather forecaster Steve Cleaton said that, while Friday is likely to be fairly calm, the bad weather will have arrived into south west England by 07:00 GMT on Saturday and will spread across swathes of the UK by the evening.
“Prolonged spells of rainfall are anticipated to gradually spread from the south and affect large areas of the UK through the course of Saturday, and during the first part of Sunday, causing flooding to areas of the country that have already been badly affected by adverse weather conditions seen earlier this week.
“In addition to concerns over rainfall, a relatively short-lived, swathe of gale to severe gale force winds is also anticipated to affect parts of southern England through Saturday evening and overnight into Sunday morning.”
Our forecaster added it was not yet clear which areas of the country would be hit by the most “significant” weather.