Peter T. Daniels wrote:

In 1992 I bought some pound notes in Northern Ireland, because I would
be staying a few days in London after my speaking engagement in
Dublin. I was surprised to read on them that they were specifically
Northern Ireland currency, even though the were issued by the Bank of
England and legal tender throughout the United Kingdom (they said.)

"Legal tender" is one of the most misunderstood concepts going. It does
*not* specify which coins & notes must and must not be accepted by
retailers. It merely applies to the repayment of debts - if a debtor offers
payment in legal tender the creditor cannot *refuse* it without
relinquishing the debt. (A semi-practical example is of tax & fee protestors
eventually repaying with wheelbarrow loads of pound coins as a final gesture
of defiance.)

Retailers are free to accept and reject whatever notes and coins they like.
Examples include:

* A lot of shops don't accept the £50 note whilst some others will only
accept it above a minimum purchase
* A number in tourist areas and airports will accept major international
currencies but with varying policies on precisely which ones and how high a
denomination
* A lot of shops in Northern Ireland accept the Euro to attract crossborder
traffic from the Republic

....and when particular note & coins have been phased out some shops have
been more willing to accept them past the official withdrawal date than
others.