It's best when they take it without really looking at it and just assume it's a fiver
Happened to me once. Gave me £4.30 change. I left him the £4, and took the the 30p. Presumably it was when he was putting the change back in the till that he realised and started chasing after me shouting "we don't accept one pound notes!". But you just did... I kindly pointed to a notice on the wall advising that all sales are final, and he eventually decided to go with it.
I don't get £1 notes very much. Even at work, £100s are much more common. The only real exception to this was when I opened my till and found ten brand new, consecutively numbered £1 notes in there just a few years ago. Dunno if it was a customer, or they'd just found an old pile of them at the back of the safe.
Just to give people an idea of how rare they are, they were last printed about a decade ago in 2001, so all the notes are at least 10 years old and most of them probably destroyed, or in the hands of collectors. I've always wondered if I could still get them at the bank on request; would be fun to go down to England with them again

I've actually got a small was of States of Jersey £1 notes I should probably try out at some point (although one's a commemorative note that might actually be worth something).
Interestingly (to pull this back on topic), Scottish banks never issued anything smaller than a £1 note. All the 10s notes were apparently issued by the Bank of England so, there ya go.