https://www.hse.gov.uk/fireandexplosion/petrol-storage-club-...
Also it goes off after like 6 months if you are not using it.
https://www.rac.co.uk/drive/advice/know-how/does-fuel-go-off....
Even above 275L, it says you need a license - it doesn't say it's illegal.
Fuel does go off, but if you have reason to store it, you probably have a use for it, so as long as you cycle the fuel, you can store an emergency supply indefinitely.
Its commonly said 'it illegal to possess plutonium', but obviously someone does operate nuclear powerplants, and thise are people with the right licence. Same applies to drugs, explosices, etc.
The question is - are requirements of the lisence realistic for you to meet as an average homeowner?
However, I picked a random Council to see, and it looks extremely doable for a homeowner with the space to do so to get a license to store up to 2,500L. It's not even particularly expensive (£44 pa)
http://www.suffolk.gov.uk/community-and-safety/suffolk-tradi...