If you dig into the guidance linked from the above page the Petroleum Licensing Authority (one or another of the tiers of Local Government) can impose conditions.

All fuel stations I use regularly have signs restricting quantities that can be dispensed into containers along lines mentioned above. Pretty sure the signage refers to regulation/licensing.

The regulations actually seem to refer only to petrol, the guidance linked from Ethan's HSE page imples as much. Site operators are likely to adopt one regime for both fuels to avoid complication - how many users want/need more than 5/10 litres for use when losing at fuel light bingo?

At one time we had a diesel only truckstop locally which also had retail pumps for cars. IIRC they had a different regime for containers but as it closed 20 yrs ago the detail is lost.

Here are the rules for jerry cans.

www.hse.gov.uk/fireandexplosion/owner-petrol-stati...m

And they are not illegal.

HSE needs to tell filling stations then - many of them still have the old limits posted.