Take a look at these guidelines from the RYA:
For work I regularly (once a every 2/3 weeks) go to the local garage and fill up with upto 250l of petrol in a mixture of 25l outboard cans and 5 and 10l plastic cans. I carry the cans in a small box trailer, the type you see in halfords, which has a locking lid. The fuel is also stored in the trailer at work if it is not in the boat. I have a fire extinguisher mounted on the front of the trailer and carry a sand bucket incase I spill any.
We have an account at the garage and the manager and most of the staff know me now by now. When I get there I go in and give them my account card and then go and fill up. This saves me having to go in each time the pump stops at 100l. The cahier just swipes the card and resets the pump and I go in and sign all the slips when I've finished.
I did have a run in with a new cashier when I went in one sunday evening when he saw me filling cans out of the trailer and stopped the pump after 60l. I went in and he got arsy with me despite me explaining that I filled up there all the time and the manager had agreed to it. He refused to come out and look at the tanks or call his manager to discuss it with him. I went in the next day and spoke to the manager who must of kicked the guys ass as he was nice as pie next time he served me!!
I think half the problem is the cashiers/managers don't know the laws about what you can and can't fill up so just get funny when people are filling up anything except a car or 5l can. I have however the been at a garage and the cashier rightly stopped a woman filling a lemonade bottle with petrol. She took it off her and then went and poured it into her car when the woman drove off!!!