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.
A more comparable situation would be if German shopkeepers had a habit of questioning French euro coins and regularly refused them by saying they’re valid only in France (even if that statement is wrong).
They may legally be worth the same, but the shopkeeper can choose to not accept them because they have the right to accept payment in any way they want. Just like a shop can refuse to accept a 500€ note when you’re only trying to buy a 1€ chocolate bar.