England:

https://flood-warning-information.service.gov.uk/long-term-flood-risk

Sign up for free flood alerts from the Environment Agency for your home or business

https://www.gov.uk/sign-up-for-flood-warnings

The Floodline telephone number is 0345 988 1188

Northern Ireland:

Flood risk, preparation and what to do in an emergency:  NI Direct - Flooding in your area

Scotland:

Live flood warnings, flood risk maps etc:  Scottish Environment Protection Agency (SEPA)

Support for community action groups, and much more: The Scottish Flood Forum

Wales:

Live flood warnings and river levels, flood warning messages and other information:  Natural Resources Wales