Lifeboats and military helicopters have rescued hundreds of people in northern England and a policeman was killed as torrential rain flooded homes, washed away bridges and closed roads.
Police in the Lake District county of Cumbria said on Friday they had found the body of the officer who was swept away when a bridge collapsed, Reuters reports.
Prime Minister Gordon Brown praised the efforts of Constable Bill Barker who died trying to save the lives of others.
Mr Barker, 44, a father-of-four who was due to celebrate his birthday on Saturday, had been directing motorists away from the bridge when it caved in.
The Environment Agency said it was a "1-in-1,000-year" flood and 314 millimetres had fallen within 24 hours in one area - a record for England.
The Meteorological Office said the amount of rain expected for all of November had fallen in one day.
Britain has been hit by severe flooding in recent years, raising questions about the impact of global warming. In 2008, the country saw its wettest summer since records began in 1914.
Police estimated about 1,000 homes in Cumbria had been damaged by the latest floods. The government has promised emergency help for affected areas.
The Met Office said Friday's forecast was better but another 40mm of rain could fall on Saturday. Officials issued four severe flood warnings, meaning extreme danger to life and property, in northwest England, and two in Scotland.
In Ireland, the Defence Forces said it was deploying 110 soldiers, 15 trucks and four boats to battle floods in the south and west of the country.
Many homes and businesses were flooded in the country's second city, Cork.