You can fill upto 20l (correct me if I'm wrong; it's either 20 or 25) as long as it's in a metal container marked flammable etc. at a roadside station.
What is all this Health and Safety, namby-pamby, PC police stuff?
I regularly take my 4x4 into a petrol station and fill it up and 2x25l, 4x10l and 8x5l cans, no problem! Well, one old biddy tried to stop we once. Just made a low growling noise at the back of my throat and showed her my teeth and she soon backed off!
Only problem I have had is, the blasted pump clicks off at about 80l and you have to go back in and ask them to note the details and reset the pump. You get "OOh, you've got a big one Sir!". To which I usually reply "Yeah, good to see you too, now reset the bloody pump".