What does one do about venting jerrycans? The pressure difference is quite significant. Obviously the vent needs to be at the top otherwise fuel will come out.
I know of pilots who fly with unofficial ferry tanks, well outside the UK. It is a huge mission capability improver because so many airports don't have avgas.
One case I know uses a thick plastic tank in a wooden frame. It was properly made for the job, and is both vented and "pressurised" by a second pitot tube. I am not sure why it needs the pressure because the plane is low wing and the tank (about 20USG) lives on the back seat, so it is higher than the wing tanks. Obviously the tank does need an external vent because it needs a vent and you don't want fumes in the cockpit, but the reason for the positive pressure is not obvious to me.