London has a desalination plant?!?!?!

Themes water is building a desalination plant to supply water to London. I can't believe that in a country where it rains every day that they really should need one.

I can almost understand countries that don't have a large rainfall or a reliable rainfall going down the desal route but there is so much water in the UK I just don't understand how they justify the energy use.

BBC 4 fontier program