There was, and perhaps still is, a problem in England where instead of carrying one's shopping home, people would just take their shopping home in the trolley and then never return it. Consequently, shopping trolleys were often found dumped. More often than not in rivers. To stop this, many supermarkets now require people to put cash in a coin slot in the trolley in order to use it, which is refunded when the trolley is returned to the supermarket (much like the system you have for luggage carts at US airports).
This is not needed in American supermarkets. It is not needed in American supermarkets because unlike in England it is not physically possible to walk to the shop. Sidewalks simply disappear. Consequently there are no coin slots but there is lots of American-made global warming from all those Americans having to drive to get their groceries.
You only really get why the world is warming up when you come to America. All those cars. All those people unwilling to walk or take the bus. All those people thinking they are entitled to drive.
Here the best (and cheapest) local grocery stores employ clean cut, articulate high school students to reload one's trolley after the groceries are paid for, push the trolley to the customer's vehicle, unload the groceries into the car, and then return the cart to the store. They will also carry groceries to the local bus stop for people who arrive by bus. All very civilized, and it is a pleasure to see young kids (14+) develop good communication skills and a work ethic. These stores are closed on Sundays, so Ian may have an issue with that. But this practice is certainly one of the reasons I shop there.