Earlier quoted context omitted.
Pretty sure this is completely incorrect. It's sterling just with a slightly different note design including the issuing bank name. It's accepted anywhere sterling is accepted - the only issue I've ever had is that English businesses may not have seen it before and are worried it's fake. Never had any refuse to accept it though.
I've had Scottish bank notes refused by a taxi driver in London, and he had a letter from the Bank of England claiming he didn't have to take it (a fact that Wikipedia backs up). Though what he gains from antagonising customers I'm not sure.
Since (presumably) the journey is over, he can insist that I pay my debt in legal tender - but not on the spot: legal tender is for when it reaches court. So, I would tell him that he can take my Scottish money or invoice me.