England, Scotland, Northern Ireland, Isle of Man and the Channel Islands all have their own forms of cash. Most of the time they'll be refused if you try to pay for them outside their locale. I've never been able to spend a Scottish note in England and the article points out they're not even legal tender in Scotland.
Similarly I've never been able to spend a £50 note anywhere. They've all been turned down.
If you try to pay a largish sum in coins most places will refuse you.
As long as two individuals agree that they're exchanging value it don't matter what the legal definition is.
Many people assume that legal tender must be accepted for payment, but that is not correct.
Legal tender has a very narrow and technical meaning, which relates to settling debts. It means that if you are in debt to someone then you can’t be sued for non-payment if you offer full payment of your debts in legal tender.
Spending money in a shop is not the same as paying a debt.