Wait until you hear about the Alderney pound, Manx pound, Jersey pound, Guernsey pound, Falkland Islands pound, Gibraltar pound, Saint Helena pound, …

which ones are accepted where is… complicated: from wikipedia:

Throughout the UK, £1 and £2 coins are legal tender for any amount, with the other coins being legal tender only for limited amounts. Bank of England notes are legal tender for any amount in England and Wales, but not in Scotland or Northern Ireland.

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Bank of England, Scottish, Northern Irish, Channel Islands, Isle of Man, Gibraltar, and Falkland banknotes may be offered anywhere in the UK, although there is no obligation to accept them as a means of payment, and acceptance varies. For example, merchants in England generally accept Scottish and Northern Irish notes, but some unfamiliar with them may reject them.[142] However, Scottish and Northern Irish notes both tend to be accepted in Scotland and Northern Ireland, respectively. Merchants in England generally do not accept Jersey, Guernsey, Manx, Gibraltarian, and Falkland notes but Manx notes are generally accepted in Northern Ireland.[143] Bank of England notes are generally accepted in the Falklands and Gibraltar, but for example, Scottish and Northern Irish notes are not.[144] Since all of the notes are denominated in sterling, banks will exchange them for locally issued notes at face value,[145][failed verification] though some in the UK have had trouble exchanging Falkland Islands notes.[146]