I went to Belfast for a meeting yesterday. I bought a cup of tea in the airport while waiting to fly back last night. I was given a £10 sterling note issued by Ulster Bank and a £5 sterling note issued by Bank of Ireland in my change. I thought nothing of it and put them in my purse.
Today, I was told by Tesco that I couldn't use the notes in the store as "they aren't legal tender, they are foreign currency". When I pointed out they aren't foreign currency, they are sterling notes, they responded "they aren't the same sterling. They are Irish sterling".
.
When I pointed out that Ireland uses the Euro, not sterling, so they couldn't be Irish, they asked where I had got them. I said "Belfast". They said, patiently because by now they think I'm an idiot, "that's in Ireland". 
I said that Belfast is in the island of Ireland, it's not in Ireland, it's in the UK. The Tesco chap said "whatever, it's still foreign currency".
AIBU to think that I'm right here?