Legal tender has a very specific meaning in law and does not in itself govern the acceptability of banknotes in transactions. Whether or not banknotes have legal tender status, their acceptability as a means of payment is essentially a matter for agreement between the parties involved.
Technically only Bank of England banknotes have legal tender status, and only within England and Wales. In Scotland and Northern Ireland BoE banknotes are promissory notes only.
Scottish and Northern Irish bank notes are issued by retail banks rather than a national central bank, and as such are also promissory notes rather than legal tender even within their home country.
There is a general agreement between banks within the United Kingdom to treat all above mentioned banknotes as legal tender however, as per the opening paragraph, that does not mean that a retailer must accept these banknotes as payment.