I'd definitely walk out if I saw that a restaurant served British food.
There’s a such thing as British Food? Who knew? And here I thought Kidney Pie and Spotted Dick were medical conditions or movie jokes...
Remember Britain and England are not the same.
So you’d be boycotting Scottish, N Irish and Welsh food. All of whom make and produce wonderful food. And to be honest, English food (and I say this as a Scot) isn’t as bad as myth makes it out to be.