“Hey that’s legal tender” Why is it so hard to use an NI bank note in England?

Righteo Sluggerverse, I need your help to answer some questions for me and I want to promote an issue.

I regularly make trips back and forth to London to visit my Mum and each time I come over I am faced with the usual problem of running about to get to an ATM that dispenses Bank of England notes or secretly cursing the cashier at the City Airport bar who gives me a Bank of Ireland fiver just before I board the plane for Heathrow.

When I get here I am always plagued with the problem of getting rid of my Northern Irish bank notes, yet they are all part of the same currency and backed by the same central bank. For me it is one of most infuriating experiences of going to London, since we are part of the same monetary union,  why should  folks living in Northern Ireland be subjected to this nonsense just because some people cannot be bothered to accept our local bank notes.

I could let this pass if we were as picky about accepting Bank of England notes or other regional currencies.

So, my question for you all is this; Is this something that just impacts me when I go over to London, or are there a lot of places that will accept our notes?

Why is it so, that there is this problem of using NI bank notes in England?

Answers on a post card please, but as this Michael McIntyre sketch shows it is something that impacts our Scottish counterparts too;