What is the legal min a car can carry of petrol in an approved container? If you want to carry in excess of that what is the process?

Mid-term lurker, first time poster.

If you're doing it for work reasons, have a look at ADR Small Load exemptions. It'll state the equipment requirements for a vehicle, training requirements and quantities you can carry. http://www.hse.gov.uk/cdg/manual/exemptions.htm#smallload

There's a fair amount of information to digest but it's doable with a couple hours, a couple phone calls to DG transport training houses and a couple hundred pounds set aside for fire extinguisher, drain cover, hi-vis jackets, shovel etc.