Oven bottoms are flat(ish) round soft bread rolls with the middle indented very slightly. They use up the space in the bottom of the oven when the baker is baking the loaves, hence the name. They were originally sold cheaply, I guess, as they were more of a by-product of the baking but in my local area became very popular.

As I said at the beginning, there was never any need for the names of bread items to travel very far, so it is probably very local to my part of North Manchester.

Yes I believe that the well-known fast food chain does sell "English Muffins" and puts unpleasant things on them.