On a related note if it's petrol in those cans you are limited to the quantity you can keep on/in a vehicle within 6 meters of a building, http://www.hse.gov.uk/fireandexplosion/petroleum/faqs.htm
Apparently there is no limit for diesel (other than personal fortune obviously :-P )
The rules are different for business and private but it's quite interesting to see what quantities are allowed, for example 333litres of petrol in a vehicle with just a sticker and a fire extinguisher....