Often the advice is to keep a little amount of money if you know you are returning, especially some small denomination notes / coins.
However, this year a new plastic £5 note (Churchill) will be start to be issued on 13 September. The withdrawal period for the current Fry note is unusually short. It will cease to be legal tender in May 2017. I think the reason for this is that this series of notes are all smaller than the current ones, so the Fry note needs to be got out of the way before the new £10 (Austin) note is issued in the summer of 2017.
Of course like all Bank of England notes right back to 1694 any old fivers will be exchangeable at face value at the Bank indefinitely.
Details on the new note: http://www.thenewfiver.co.uk/