In Scotland and Northern Ireland, Bank of England notes are not legal tender, but are widely accepted. The reason for this has its roots in the past. In the early 18th century, both Scotland and Northern Ireland had their own currencies, the pound Scots and the pound Irish, respectively. These currencies were pegged to the pound sterling at a rate of 12 pounds Scots or 13 pounds Irish to 1 pound sterling.
In 1707, the Kingdom of Great Britain was formed, and in 1801, the United Kingdom of Great Britain and Ireland was created. However, despite these unions, Scotland and Northern Ireland retained their own currencies. It was not until 1928 that the pound Scots and the pound Irish were demonetised, and replaced with the pound sterling.
Even though Bank of England notes are not legal tender in Scotland and Northern Ireland, they are widely accepted due to their universal recognition and ease of use. However, shops are not legally obliged to accept them, and some small businesses may choose to accept only their local banknotes, due to higher charges for depositing Bank of England notes.
In conclusion, Bank of England notes are universally accepted in England and Wales, but are not legal tender in Scotland and Northern Ireland, although they are widely accepted in practice.