I ve had similar problems, especially at Tesco when filling the outboard tanks for the boat - 25ltrs each.
If they are on the boat no ever says anything but if I try to fill them in the trailer or the boot of the car people start asking questions despite the fact they meet the regulations.
Found one way of doing it is to stop the pump every 5 ltrs and pretend to move to another can in the boot. Sneaky but easier than taking a 5ft boat into Tesco!