They cant effectively enforce the rule, but they have to set a limit somewhere!
Depends on your neighbours - my mate had an IBC of oil, and his neighbour stuck his nose in a complained to somebody. I think its nigh on impossible to prov how much oil you are actually using though - he only had a few hundred ltrs
You can get whats called a dual tank system, which uses your main diesel tank to store the veg oil. mYou then have a small jerry can sized fuel tank somewhere else, full of diesel. When you start your engine, the diesel is used. After a few mins, once the engine is warm, the oil is used, having been fed through a heat exchanger, to thin it. oil is then used throughout the journey. A few mins before you reach your destination, you press a button on the dash, which brings the diesel back into circuit, to "flush" your fuel lines clean of the oil. This totally alleviates the problems of thick cold gloopy oil congealin in fuel lines, and your poor fuel pump having a coronary, trying to move said oil. No more injector issues, as they only pump warmed less viscous oil, or diesel.
Kits are anywhere up to a grand, but at least you wont be replacing injectors, fuel pumps etc.