I think....
From a quick search:
Diesel:
Diesel carried by private individuals is not regulated as regards how much may be carried and how. Anyone contemplating carrying diesel in a vehicle should bear in mind the potential for fire and the likelihood of spillage from inadequate containment.
Petrol: Regulations made under the Petroleum (Consolidation) Act 1928 control the quantities of petrol permitted to be kept in containers for private use. This limit also applies to carriage. A maximum of two metal containers each up to 10 litres capacity, plus a maximum of two suitable and appropriately-marked plastic containers each up to five litres capacity, can be kept in a motor vehicle.
For further information on portable petrol containers see the HSE website
www.hse.gov.uk/lau/lacs/65-9.htm or contact your local Trading Standards Officer.
The problem might be the rules of the ferry company or the train compnay (if Eurotunnel)
Ian