Despite Scotland being considered as part of the United Kingdom, they print their own currency which issued by the Bank of Scotland (but the confusing part is that there’s no separate currency code for Scottish Pounds) and the fact it is “legal currency” but not officially legal tender (even in Scotland itself), it’s weird.
I would say that using GBP is better than Scottish Pound to not confuse cashiers since it’s up to their discretion whether they’ll accept Scottish money in Britain & vice versa. I’ve heard most will refuse Scottish Pound due to unfamiliarity, even worse there are three banks printing their own version of Scottish Pounds.
The six Scottish and Northern Irish issuing banks buy sterling currency from the Bank of England using the largest denomination banknote, the Titan (£100 million). This is the only note printed in-house by the BoE. The issuing banks can then print notes up to the value of the titans held in their BoE account.
This used to be common throughout the UK, but the last English banks gave up their issuing rights in the C20th leaving the monopoly in England & Wales with the BoE. Scotland has a particular interest in having their own notes as they invented banknotes, and while they used English notes after Union, a 19th century poetic revivalist movement (also responsible for kilts and tartan) led to the reintroduction of the Scottish notes.