Fair enough. I've always considered that any book which portrays a different world to the one we live in counts as SF, regardless of whether or not it has science in it (unless it has elves and magic, in which case it's probably fantasy
).
A good example is Stephen Baxter's "Northland" trilogy, which describes a world in which the course of history in Europe and Asia (and ultimately the entire world) is completely different to that of our world, all because of a decision made by one woman around 5000 BC. There's no science in it; it's essentially a trilogy of historical novels - but not our history. Nonetheless it's published by Gollancz SF.
Hence I would also consider a novel set in Roman Britain with zombies to be SF.