When do the old £20 go out of circulation?

Have you heared when the old £20 go out of circulation? I have a bunch of old £20s and want to know until when can I use them. After the £20 expire how to get rid of them if I find some left? Do the banks exchange them for new banknotes? Until when can I take the expired money to the banks and which bank exchanges the old notes to new ones?