Some people couldn't afford this, so would put in foreign coins or other coin-shaped objects. My dad would come across these from time to time while he was emptying the meters. He had the awkward job of asking the household to cough up in actual money. Fortunately, they usually could because the meters were emptied in summer when they weren't paying for as much fuel so had some money. The acceptable, face-saving excuse was that they didn't have the right coin when they needed one, so would pay the difference in notes and my dad would give them the change in the coins they were short of. He didn't mind this, because coins are a lot heavier than notes.
As for the foreign coins, the gas board didn't want them, they just wanted the people not to use them again, so my dad would bring them home. We used them as tokens when we were playing gambling games (most often Newmarket, but also Three-Card Brag) (aside: my wife comes from the town of Newmarket; there isn't a town called Three-Card Brag).
My favourite such coin was this one:
When I first saw it, I thought it must be 600 years old and worth a fortune. It soon occurred to me that if it were 600 years old and worth a fortune then no-one would have put it in a gas meter instead of a shilling. Why was the date 1371, though? Was it a forgery?
It was this coin that first introduced me to the fact that not all countries record dates the same way. I knew about the difference between Julian and Gregorian calendars, but not that the Islamic world used a different system entirely. 1371 in the Hirji calendar ran from 2nd Oct 1951 to 20 Sep 1952.
So, not 600 years old and worth a fortune, then.
I don't know how this coin made it into the possession of someone in a small town in East Yorkshire, but my guess is that someone brought it home from when they were doing National Service. Most of the coins are from places in Europe and probably originated with either children who came home from school trips, people who worked on the ferries from Hull, or foreign visitors who made the mistake of visiting the area.
I may subject you to a few more posts about some of the other coins my dad found, if I get round to it. None of them are worth anything beyond their base metal value, by the way: people who know about coins have looked through them and been disappointed.
Not everyone who cheated gas meters used foreign coins. Well-to-do people, who had freezers, would construct a mould out of plasticine and make a coin out of frozen water. They thought that this would leave no evidence, so it wouldn't look as if they'd been stealing gas. Unfortunately for them, this practice rusted the coin box and thence the whole meter, which they'd have to pay to have replaced; it cost them a lot more in the long run, but at least they felt clever for awhile.