Some can take payment over the phone if you can ring someone. Otherwise I would imagine you leave driving license or something else to collect later.
If you were to just leave the station against their will and then come back and pay, would you have committed an offence?
Law seems to say:I would offer that if you leave with intent to collect the means to pay, so you aren't acting dishonestly and you do not have intent to avoid payment, you aren't guilty of an offence.
Haven't done what's in the OP but I hate it when I fill up with a certain amount of fuel i.e. £40. I try so carefully to make it land on £40, however, it lands on £40.01. FFFFFUUUUUUUUU.
One of my uni friends has a good game with this. His current game works as follows...
Have 2 x £20 notes. Fill with £20 and go a "little" over. Say £20.01
Offer to pay with the 2 x £20. More often than not, they'll let you off.
He's currently got up to £20.36 and got away with it.36p worth of "free fuel"
I keep reminding him that he's the reason why fuel prices are so damn high!