You are allowed 2500 liters per annum without paying duty. I use bio diesel in my truck and top up with diesel, as I can only get 25 liters per week from my supplier. He recycles used sunflower oil from the local crisp factory, and this goes through, I believe at least two filtration systems, then heated to a certain temperature to remove any water
Then the last process is to have the glycerin removed.
Using neat veg oil, unless diluted with mineral diesel, can gum up the injectors. Some people add a liter of unleaded to dilute it even more
This Linky might provide an insight to the use of Veg oils