Funny you say that - I had just come back from Edinburgh back to London and went to the post office trying to mail something, went to the counter and was trying to pay by cash as I had a few notes from the trip, the lady behind the counter sees they are Bank of Scotland notes and says, "do you want to hand them over dear? I will exchange them for some real English money. People don't really accept them around here!" I had a massive chuckle. My first exposure to the varying currency laws in the UK!
I did wind a mate up many years ago who was going to work in Belfast. I told him to be careful which money he spent. Ulster Bank in the Protestant areas and Northern Bank in the Catholic areas. I also told him about the special currency exchanges on the peace wall...
(It's 20 years since I've been to either place so the banks may have changed since then.)