You can legally fill a 5 litre can provided it is correctly marked petrolium spirit highly flammable, most new ones are plastic, but a steel one would be OK, but not an old oil can.
Some garages do not like people filling cans because they are more likly to run off wthout paying and/or are up to illegal activities, but it is still legal. I reguarly fill carrect plastic cans for chainsaws with no problem, but then I am in a signwitten van.