Alot of trackday and offroad enthusiasts (and indeed, members of general public) have been querying the amount of fuel that they are allowed and what type of containers are accepted. This should hopefully answer your questions!
How many containers can I fill at a petrol station?
Petrol filling stations may have their own internal policy on the types and numbers of containers they are prepared to fill - frequently one or two 5 litre plastic and/or one or two 10 litre metal. This is a decision made by the filling station operator and is not a legal requirement.
Petrol filling stations usually have to abide by a licence condition to allow only 'suitable' containers to be filled. This is usually interpreted as metal containers up to a maximum size of 23 litres or plastic containers up to a maximum size of 5 litres. A licence condition has the same effect as a legal requirement. The licence condition does not limit how many containers one customer may fill.
Are there any restictions on the transport of petrol?
The Carriage of Dangerous Goods and use of Transportable Pressure Equipment Regulations 2004 (CDG), as amended by The Carriage of Dangerous Goods and Use of Transportable Pressure Equipment (Amendment) Regulations 2005 require that petrol should be carried in UN approved containers (called Packages), which are properly stowed on the vehicle.
The packages should be marked with the "flammable" diamond and with the UN number for petrol (UN 1203). Up to 333 litres may be carried as a "small load" which means that only general training needs to be given to the driver and that the vehicle only needs to carry a 1- 2kg fire extinguisher. These regulations do not affect purely private carriage.