Silly answer, but it works : You go to the petrol pump, and fill up your bottle. If the attendant is arsey, and cuts off the pump before it starts dispensing, you give up and go to another petrol station. Fortunately in places where petrol stations are few and far between, like the scottish highlands, the people are also much less likely to be miserable gitbags. Even more fortunately I've not heard of anybody having had that happen to them.
The "minimum 2 litres delivery" is also something which can safely be ignored. A friend did get to the point of saying he'd filled his bottle, what did they want him to do with the rest, pour it onto the forecourt, but they relented and sold him the small amount.
The same applies to things like ferries - I was very surprised to see the Isle of Wight ferry company people saying petrol bottles weren't allowed for their incredibly short crossing. So they just go in the pannier where nobody looks.
Air travel is harder - for that, I washed the bottle out. It got rejected as hold baggage, but after another wash with perfumed soap, it came through as hand baggage with no problems. "What's that?" "It's a washed out petrol bottle with no lid" "Ok". I think the lack of lid was a help too.
All of this is a big reason why I have a petrol stove - fuel can easily be got almost anywhere for reasonable prices.
