There is no actual law that states the size of portable metal fuel containers, only that they must be UN approved. However, the HSE guidelines (which might as well be law!) state that you can use plastic containers up to 10L and metal up to 20L. It's only petrol stations that can determine how much they will allow you to fill, as per their own RA.
Oh yeah, and you can only carry 60L in your car too in portable containers. That's a lot of fuel though!