You can unscrew the keyboard, gently pop the keys out and wash all the non-electrical parts with warm water (this is what I used to do when I was a poor student), but as Padge rightly says, they only cost about a fiver, and you can buy covers for them to stop more dirt getting all over them, so I would do that.
Remember when you're buying a cheap keyboard to check that the keyboard arrangement is for UK so, as some of the really cheap ones might not be, if it's got a pound (☼) sign, it's probably alright.
Don't know if that pound will come out as it doesn't feature on this Dutch version
Hope this helps
David