"... local High Street shops and banks no longer have to take them after today."
Logal High Street shops don't have to take any money they don't want to. No Fifties, No Scottish Money, No Coppers --- they could even refuse to serve people who want to pay in cash. Or even Stirling.
Perhaps your local corner shop would have to take it if you were paying your newspaper bill or settling some other account. Legal tender is that which, when offered, must be taken towards settlement of a debt. So perhaps a restaurant like Pizza Express, where you sort-of incur a debt before paying? But not Nandos, where they make you pay up front, to make sure you can't object to the bill.
PS: Handy hint: if you get you and your date a McDonalds Peri-Peri chicken wrap meal each, and set fire to a twenty, it's exactly like eating at Nandos.