I don't think I'd be exaggarating if I said that 99% of people in the UK use butter/margarine on sandwiches.
The only exception I can think of would be Jewish people keeping kosher, or people who are watching their diet very carefully. Even the vegans I know use vegan-friendly margarine! Actually, Jewish folk here probably use that too.
There must be some people here who don't out of preference, but I have literally never met them. Of course, mayo is much less common here.
How can you have a chip butty without butter? Answer: you can't, because that would be a travesty.