And how is it their problem if they have that note due to being handed it with change?
as has been said, once the note is in your hand, and you put it in your pocket, your stuffed,
in fact that's the other trick they use, go into a shop with a legit £50 pay for something like cigs, then they have two legit £20 in their hands, they palm them and swap for fakes then complain, asking for 'real' £20
on a side not, the easiest and most effective way of spotting a fake, is to run your thumb across the 'Bank of England' at the top of the note, they have never been able to fake the raised text.