There are plenty of dams already in the UK and also plenty of water but not necessarily close to th egreatest centres of population in the south east.
More thought needs to go into water supply and treatment and sewage and waste disposal when giving planning permission for new housing or industry. All houses should be metered and have water butts or cisterns to collect rain water for gardens and toilets. Water companies should be forced to make repairs to reduce water losses through leaks - up to 40% in some areas!
Flood plains should not be built on - it's asking for trouble - and areas like the Somerset levels should have their drainage channels and waterways regularly dredged to reduce flooding near homes and businesses.
None of this is rocket science. Just common sense and good management.