Living in Berkshire, I suspect that the problem with accepting non-English bank notes is the attitude of other customers. A shop keeper that accepts one of these notes effectively has to put it aside for taking to the bank and cannot easily use it to make change. This is because customers in the south of England are reluctant to receive Scottish pound notes, especially if there are English notes visible in the till. I suppose I see around 5 Scottish notes a year and have never seen a Northern Irish or Manx one.