For some reason I dislike doing this. When you're putting your trolley back and people try to give you their £1 and you give them your trolley.
I don't want their £1, I want to keep mine.
It's happened today and the lady gave me 5 20p coins, in exchange for my £1 coin, so essentially I just gave her change. Only now I don't have my £1 coin (which I always keep in my car for the trolley), so I've now got to change it myself back to £1.