People often use the term “Biodiesel” when talking about Vegetable oil.
And they are not the same thing at all.
Biodiesel (B100) is basically diesel that’s been made from vegetable or animal fats. Can be used alone in any Diesel engine without any modifications.
Vegetable oil, unused or used (SVO or WVO) is different altogether.
As you can buy proper B100 Biodiesel for £1.04/litre, and the price of SVO isn’t much lower, it seems like a lot of hassle to use SVO to me.
If you can get WVO for free, it will need filtering and de watering before it can be used. Which is fairly messy, but worth it as if you can do it right, it’s essentially free fuel (other than your time and a few materials and a bit of electricity.) other than your set up costs obviously.