It can be used anywhere. However, due to the fact that it isn't legal tender, it can be refused when presented to settle a debt.
Same with Scottish and Northern Ireland pound sterling notes - can be accepted as payment for a debt anywhere in the UK, but as it's not legal tender it can be refused.
Credit cards, debit cards, cheques and other non-cash methods of payment aren't strictly speaking legal tender (i.e. it's totally legal for the person who is owed the money to refuse it)