I think if you drill down into the legislation it actually requires one to declare "Intended Use", i.e. the use in the future of what you're buying now. Not what the current batch already in the tank is being used for.
So if you intend your next purchase of fuel to be 100% domestic, because the tank is currently half full of say 60/40, who is in any position to prove you wrong about your intentions at the point of purchase?
You might however, change your mind later...