(Not a lawyer but from my 1 module at uni) I belive that you are correct on both counts. Scottish bank notes are NOT legal tender anywhere and on one has to accept them except the issuing retail bank.[1]
So retailers can reject them for the same reason as when you see something labelled with the wrong price. The shop keeper is under no obligation to sell it to you for that price.
What you actually see is a reference for you, the customer, to initiate an offer to buy something at that price.
If the shop keeper doesn't like the look or you money or the price your offering he can reject the deal.
As far as legal tender goes that only applies to the settling of debts and a retailer could reject an offer to sell they don't like even if you do offer legal tender.
Even Bank of England copper coins are only legal tender up to something like the value of 20p. So you can't take a truck load of copper coins to your landlord to pay for rent in arrears.
1. https://en.wikipedia.org/wiki/Banknotes_of_Scotland
