Hundreds of homes were swamped by floods along the River Thames while British Prime Minister David Cameron warned that stricken communities were "in it for the long haul".
The floods played havoc with rail travel, with one of Britain's busiest routes from London to the commuter town of Reading heavily disrupted.
Affluent towns and villages along the Thames to the west of London have been transformed into lagoons, and the government faced renewed criticism that it was under-prepared.
More than 1,000 homes have been evacuated along the Thames, in villages and towns such as Wraysbury, Datchet and Chertsey and the situation was set to worsen with heavy rain and storms on the way by Friday.
Flooding first hit the largely rural southwestern county of Somerset but has now engulfed towns and village along the swollen Thames in the southeast, encroaching on London.
A total of 1,600 troops are on standby to help, and some were already at work filling sandbags in Wraysbury, where one resident had a bitter exchange with Defence Secretary Philip Hammond.
Su Burrows, a volunteer flood warden, said the relief effort had been left to residents like her and pleaded with Hammond for military help to distribute sandbags.
"I'm sorry, I am going to get emotional. There are 100 people of this village currently working together, none of them (Environment Agency) agents, not one," she told him in the exchange on Sky News television.
Burrows told AFP later that her blast seemed to have borne fruit, as 2,000 sandbags were sent to Wraysbury, followed soon afterwards by 100 soldiers.
Hammond insisted earlier that the government "has got a grip on this" but cautioned that authorities cannot "prevent the course of nature".
Cameron saw for himself the damage to the railway line at Dawlish in southwest England, caused by massive waves crashing against the coastline.
Train services have been cut off by the damage to the track.
Cameron said: "It is a huge challenge and we have had the wettest start to a year for 250 years, some of the most extreme weather we have seen in our country in decades.
"We have to recognise it is going to take time before we get things back to normal."
Insurers said overall claims had already exceeded £500 million (NZ$987 million) and the bill would rise fast.
Tens of thousands of commuters had their journeys disrupted as services from London Paddington station to Reading to the west were heavily disrupted.
Andrew McKenzie, a hydrogeologist from the British Geological Survey, warned that some communities could be flooded for months due to high groundwater levels caused by the persistent heavy rain from mid-December onwards.
Underground layers of water-bearing rock, called aquifers, in southern England were half-full before the rainy spell began, and have since seen "spectacular" rises in groundwater levels, he said.
"The very heavy rains over the New Year raised groundwater levels and we got some localised flooding," said McKenzie.
"But even at that point we weren't sure how widespread that was going to be because there was a lot of empty storage in the aquifers.
"The prolonged rain has just changed the situation totally."
He said he expected to see "many more months of groundwater issues".