The way I see it:
Legislation relating to carriage of dangerous goods does not apply to private carriage. It all refers to 'vehicles involved in work activity.'
There are rules covering storage of petrol for domestic use:
The Petroleum Spirit (Motor Vehicles etc.) Regulations 1929 and the Petroleum Spirit (Plastic Containers) Regulations 1982 limit the amount of petrol that can be kept in a domestic garage or within six metres of a building (e.g. most domestic driveways). The limit is a maximum of two suitable metal containers each of a maximum capacity of ten litres or two plastic containers (which have to be of an approved design) each of a maximum capacity of five litres.
So if you want fill up the night before, and leave the petrol in your garage, or in the back of your car, ready for the morning, you should stay within these limits.
There is a grey area though, as a 25 litre plastic petrol tank could be OK, although some will argue that once removed from the boat, it's not a fuel tank any more, just a plastic container. I keep my boat on my drive, within 6m of the house, but am comfortable that even if I have full 25 litre tanks on the boat, I'm not breaking any laws. I am not storing petrol in plastic containers, I just have full fuel tanks on my boat.
However, none of this legislation stops you from filling as many approved containers as you like, sticking them in the back of your private motor vehicle, and going directly to your boat.
Whether or not a filling station will let you do this is another thing.
They may have maximum limits they are willing to dispense into portable containers, or any fuel tank for that matter, either imposed by their licence conditions, or of their own choice. It may be that they have noticed a great proportion of people who fill portable containers run off without paying.
At the end of the day, if the filling station won't let you do it, they have their reasons, and no amount of argument will sway them, even if there's no legislation to back them up. So find a garage that will let you fill up.
Pennog.