I don't need to do this but know people who do, ye you may be liable for a fine I don't know, you may even be able to get a license I just don't know.
What I do know is if you buy a stand pipe with tap on, and the key is just a square head on a pole this turns water on and off. You can have water.
Worst case scenario, I would have no probs doing this at all, ok a little discretion but in alls honesty who is guna ring up the water board an say " I just seen a van using the hydrant" there's millions of them
A lad I know fills his van every night right outside his front door, he's been doing it for as long as I know - more than say five years