Watch out, Boo! I think that your question might upset any AB members who are from the Channel Islands :-)
Your question seems to imply that the Channel Islands are part of the UK. They're most definitely not! The Channel Islands are neither part of the UK nor of the European Union. They don't send Members of Parliament to the House of Commons and they don't have any representation in the Lords. No legislation passed by the UK Parliament has any standing in the Channel Islands unless the States of Jersey and Guernsey request that this should happen.
So, therer's no reason to assume that the currency of Guernsey should be accepted in the UK.
Anyway, 'legal tender' is a strange legal concept which doesn't really have much bearing on everyday life. For example, Scottish banknotes are accepted throughout most of the UK but they're not officially legal tender anywhere, even in Scotland!
Chris