Beware of a scam which was used on me. I went to an official Change window...I wanted to change £150 so took the notes from my purse. The assistant then passed one £20 back which was torn almost in half, I apologised and gave her another, taking back the damaged note. As we walked away I pondered how I had not noticed giving her a torn note and realised that I only ever get notes from an ATM so could not have had the torn one. Sure enough on close inspection it was a forgery, which she had very easily got rid of. Make sure you count out your crisp notes carefully.