In a Twitter message, Prime Minister David Cameron described scenes of flood in the rural southwestern region of Cornwall as "shocking", and promised that his government "will help ensure everything is being done to help".
Two people have died since heavy rains began on Wednesday, including a woman killed by a falling tree in the southwestern city of Exeter and a man who became trapped in his car in rising waters in Somerset, also in the southwest.
The Cornish village of Millbrook was reportedly under five feet (1.5 metres) of water with 40 homes evacuated, a BBC reporter who lives there said, after torrents of muddy water swept through the village on Saturday.
Many communities were cut off after police shut water-logged roads in Cornwall, where four severe flood warnings were issued overnight over rapidly rising river levels, and neighbouring Devon.
Across Britain, the Environment Agency said nearly 500 homes have flooded since Wednesday and warned more rain was on the way on Monday.
In Malmesbury in Wiltshire, western England, pub landlord Tom Hudson said he had water lapping at the door in the worst floods he had seen for 14 years.
"It's gone down a lot but I'm trying to get hold of some sandbags because more rain is forecast for later today," he said.
"Houses across the road have been flooded to a depth of three or four feet, with furniture floating around in the rooms.
"I've been here 14 years and there were floods in 2000 and again in 2007 but this is much worse than either of those."