RampantScotland describes this anomaly and provides other useful money travel advice for Scotland. Is this anomaly due to the fact that Bank of England paper currency notes are accepted everywhere in Scotland but notes issued in Scotland by the three Scottish banks are not commonly accepted in England (even though they are legal tender anywhere in the UK)? The other pages of RampantScotland contain a miscellany of useful and amusing facts about the place, well worth a wee browse.
For trivia buffs, BBC article "Scottish Money Needs Protection" explains how the anomaly came about:
"In 1826, the British parliament passed legislation preventing banks from issuing their own pound notes, a practice which was threatening to get out of hand.
But a vigorous campaign in Scotland, which enlisted figures such as the writer Sir Walter Scott, ensured that it was exempted from the new law."
Although RampantScotland says overseas banks issue only Bank of England notes, that is apparently not so in Canada as the the Royal Bank of Canada has two sets of buy/sell rates for cash and indeed you must pay more for Scottish pounds than English pounds. In addition, when you want to sell them back to the bank at the end of your trip, you get less for Scottish pounds. Today, for instance the buy/sell spread for Scots pounds is 7.165% but only 6.893% for the English kind.