A UK centric reply but there are several issues. Buying the stuff, transportation and storage, all separate.
Buying: Individual filling stations can choose to allow, or not, purchase of large volumes of fuel in a single transaction, which is really nothing more than straightforward consumer law. Local authorities and oil suppliers may also impose maximum permissible volumes for single purchases.
Transportation: There is a distinction made in the regulations between transportation of fuel by private individuals for their own use and that done commercially. See here:
https://www.gov.uk/government/upload...-2015-rev2.pdf from which;
Private carriage
Nonwork related carriage by private individuals of fuels packaged for retail sale and intended for their personal or domestic use or for their leisure or sporting activities is exempt from the requirements of CDG 2009; however, measures should be taken to prevent leakage of the fuel (ADR 1.1.3.1 (a)).
The previous version of that Guidance note also stated:
When these goods are flammable liquids carried in refillable receptacles filled by, or for, a private individual, the total quantity shall not exceed 60 litres per receptacle and 240 litres per transport unit.
Private Storage: The regulations put upper limits of 2 x 5 litres plastic containers which have to be of an approved design, and 2 x 10 litres metal containers, which should be marked with the words “PETROLEUM SPIRIT HIGHLY FLAMMABLE”.
In Scotland, which affects me, there is also a SEPA requirement that fuel receptacles be stored in a bund or drip tray capable of containing 25% of the total receptacle volume. There may be a similar requirement in other parts of the UK.
In practice, maybe follow Crash one. (It's my business, unless I make it your business.)