They were supposedly invented in Scotland, and I've not seen them sold in an English fish and chip shop. Go to Scotland however and they're not uncommon.
https://en.wikipedia.org/wiki/Deep-fried_Mars_bar
I've never seen nor heard of them anywhere in Scotland, but I have seen them in the Midlands.
I've never seen nor heard of them anywhere in Scotland, but I have seen them in the Midlands.