Sunday marked the first time the service sounded a red warning since December 2015.
John Curtin, the executive director of flood and coastal risk management at the Environment Agency, said in a tweet that at one point during the day, England had the most flood warnings and lower-level alerts in force - 594 - than on any other day on record.
The Met Office said the highest wind gust recorded was 146km/h at Aberdaron in North Wales on Saturday.
Residents in some parts of the country were advised to evacuate due to the flooding, while South Wales Police declared the severe weather conditions a "major incident".
Storm Dennis, the combination of two storm centres over the Atlantic, caused rivers in Wales and Yorkshire to break their banks.
The Met Office, Britain's meteorological service, only issues its highest red warning when it thinks the weather will be so risky there's a "risk to life" and that people must take immediate action to protect themselves.
A total of 156.2mm of rain fell at Crai Reservoir in Powys in the 48 hours from Friday to Sunday morning, it added.
South Wales police said they had been dealing with "multiple" landslides in addition to the floods, with some residents trapped.
Storm Inès and Dennis: Further storms expected from Thursday onwards
Several planes rocked from side to side as they came into land at Birmingham Airport during fierce crosswinds from Storm Dennis. The winds could cause disruption to travel, and there's a chance of some damage to buildings.
Severe flood warning have been issued for the rivers Neath and Taff in South Wales, as well as the River Teme further north.
You can check the latest from the Met Office here.
A status orange wind warning is in place throughout the county as winds with mean speeds of 60 to 80 kilometres per hour and severe gusts of up to 120 kilometres per hour are expected.
Some of the regions now being affected by Dennis were also hit by a preceding storm, Ciara, which brought stronger winds than Dennis but less rain.
On Saturday the storm claimed two victims in rough seas in Kent, where two bodies were pulled out of the water.
Rail services were suspended across South Wales after tracks were submerged by rain, while the line between Derby and Long Eaton was also closed.
The Met Office was predicting 140mm of rain to fall in South Wales on Sunday morning before the storm eased in the afternoon.
Continuing, the ESB statement noted: "The vast majority of customers who lost power during Storm Dennis were restored yesterday, Sunday".