is it really worth the risk

paying £1 per litre instead of £1.40 and running the risk of:

blocking fuel filters, breaking lift pumps and injection pumps.
driving around stinking of veg oil
having to fill out all the paperwork for HMRC - if required.
cold starting problems

fair enopugh if you can get used cooking oil for peanuts or free but i wouldnt buy veg oil for that purpose.