Some UV protection options from Paula Burch.
http://www.pburch.net/drupal/?q=node/505
The only one stated to be water soluable seems to be Fabric Shield. Here's more info on that one, but it can only be shipped within the US due to being an aerosol.
http://sew-craft.com/spray-and-fix/fabric-shield.html
Non-aerosols include Forcefield UV Sunblock and Rit Sun Guard (previously mentioned). They might ship from the US or be available in the UK.
http://www.amazon.com/Sun-Guard-Laun.../dp/B0000Y3F6W
http://www.amazon.com/Vogue-Fabrics-.../dp/B008EUJDCY