It's the country's largest ever civil emergency test.
British officials are preparing for the threat of increased floods associated with global warming by testing the country's response.
They say some 10,000 people are participating in the $2.9 million exercise.
The plan is to evacuate a school and then have helicopters pluck people from rooftops as part of the week long exercise announced Monday.
Some of the drill include: Praciticing for flash flooding across London and a North Sea tidal surge along the country's east coast.
In 2007 and 2009, Britain was hit with a series of devastating floods.
The exercises were among the measures proposed by a government inquiry into the flooding.
With files from the Associated Press