I don't know if they sell these in the UK, but here in the USA there's a company called Optech/USA that makes a product called Rainsleeve. It is a plastic bag with elastic on one end. You put the elastic hole around the lens, near the front of it (I put it on the lens hood so the whole lens is covered) and the rest of the bag goes back over your camera and keeps it dry in the rain. They're cheap, $7 for a two pack. I think that's about 4 pounds?
Optech/ USA products are indeed sold in the UK Chris, I use their neckstraps on all my cameras.