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 legal tender argument/debate kinda annoys me as people seem to think it has something to do with legitimacy.
“Legal tender” in England and Wales is money that the English and Welsh courts will accept in payment for debt. So the Courts said we accept money issues by the Bank of England in the follow denominations, etc.
The Scottish Courts said “we’ll accept money, or whatever we deem is acceptable to repay a debt we’ve issued” - so the legal tender definition doesn’t even mean anything in Scotland.
Keep in mind, legal tender is really specific, so if you try to dick around and pay a £1000 fine in 2p coins - it will be rejected as that is not legal tender. You can only supply certain coins up to certain amounts.
But anyway, Scottish money, or Northern Irish money is valued exactly the same as BoE issued money.
A shopkeeper being unfamiliar is fine, they should be cautious. But until they do interact with it, they’re always going to be unfamiliar.
But, this is most likely to get worse as cash becomes less common anyway.