My understanding is that the Bank of England allows the Scottish banks to print their own notes but that they are not actually legal tender, so that is why they can be refused.
Are there many cashmachines in Borovets? And do they (the machine) charge for withdrawals? I normally take some local currency out at the airport when travelling and then as I need it in resort using a Nationwide BS debit card. This is because Nationwide do not charge a fee for withdrawals abroad, and they also do not commission load the exchange rate on the withdrawals either.
Last year we took both Levs and Euro's. Levs are taken in every establishment on and off the mountain. Euro's can be used in the hotels. I found the best exchange was with M&S for Levs. Came home with a wallot full of Euro's tho'. Anyhow we will be back in Feb and already it's looking better than last year!!