Making a surface plate is kinda like hand sanding a knife blade. You start with a coarse grit and progressively move onto finer grits. Lapping is just the finest "grit" possible. Even Whitsworth probably scraped his plates flat using a "poor" reference plate and only then did he start lapping.
If you can find a flat piece of glass then use that as your starting point and then apply the 3 plates method.
I'm planning to make granite surface plates myself too. Where I live, we have granite tombstone makers in every small city and several in every big city. You can rather cheaply order any size slab you want and it's typically pretty flat too (polished).
If you can find a flat piece of glass then use that as your starting point and then apply the 3 plates method.