It is the case that Northern Ireland notes are more likely to be seen suspiciously, especially those from First Trust who have only been doing them for a few years and many have never heard of them.

I believe the Royal Bank in Scotland still does a £1 paper note, which is particularly difficult to get rid of in England where this value has for long only been a coin.

There are also Isle of Man and Channel Island notes, which are even more of a challenge in smaller shops.

In Northern Ireland there was an extremely large robbery of Northern Bank notes some years ago, big enough to cause the bank to withdraw rapidly all in circulation and issue a new and different design, and as in the UK they were rare anyway those were particularly difficult to get rid of, as people were not familiar with what was old (which were all officially cancelled) and what was new.