Blurb - Currency
While Scotland and Northern Ireland aren't allowed to have their own currency, some of their regional banks are allowed to print their own banknotes. There is a great deal of confusion about whether these notes are "legal tender" or not; most of this centres around a popular misconception of what "legal tender" actually is. To cut a long story short, retailers in England, Scotland, Wales and Northern Ireland should treat pound notes from any issuing bank within the United Kingdom as being equal. Unfortunately you're unlikely to have much of a leg to stand on whilst waving your Northern Irish ten pound note at the chap behind the counter in a Welsh McDonalds, because he's not legally obliged to accept English ten pound notes either. All these details and more are available in the Bank of England's Currency Fact Sheet.
While a great deal of mainland Europe is now using the Euro as a "street" currency, the United Kingdom is holding off for the moment.