Just to correct some of the incorrect information posted about legal tender...
Bank of England says:
Are Scottish & Northern Ireland notes "legal tender"?
In short ‘No’ these notes are not "legal tender"; furthermore, Bank of England notes are only legal tender in England and Wales. Legal tender has, however, a very narrow technical meaning in relation to the settlement of debt. If a debtor pays in legal tender the exact amount he/she owes under the terms of a contract (and in accordance with its terms), or pays this amount into court, he/she has good defence in law if he/she is sued for non-payment of the debt.
In ordinary everyday transactions, the term "legal tender" in its purest sense need not govern a note's acceptability in transactions. The acceptability of a Scottish or Northern Ireland note as a means of payment is essentially a matter for agreement between the parties involved. If both parties are in agreement, Scottish and Northern Ireland notes can be used in England and Wales. Holders of genuine Scottish and Northern Ireland notes are provided with a level of protection similar to that provided to holders of Bank of England notes. This is because the issuing banks must back their note issue using a combination of Bank of England notes, UK coin and funds in an interest bearing bank account at the Bank of England.