The below quote is from the internet: I have copied only the first part as its really boring !!!!!!
The final paragraph did say, there was no good practical reason why the notes of each part of the UK should not be accepted in all parts. But, of course that does not mean that all notes should/must or may be accepted in any other country, thus the problem in Phil and the lower rate.
( As I now live in Euroland I can't see any reason whatsoever why there should not be just one note for the 'little off shore islands' of the UK !!!!!! Of course that applies to Gibraltar too where they have their own notes, where although the English pound is accepted it is not vice versa)
EXTRACT:
NORTHERN IRELAND ASSEMBLY
RESEARCH AND LIBRARY
SERVICES
BRIEFING NOTE 122/08
THE STATUS OF SCOTTISH AND NORTHERN IRISH BANKNOTES
The Bank of England provides the following answer to the question of whether or not Scottish and Northern Irish banknotes are legal tender:
“In short ‘No’ these notes are not legal tender; only Bank of England notes are legal tender but only in England and Wales.