Ah hah! You've hit on my rule for judging what kind of a person someone is. Because there is absolutely no consequence for not returning the cart to the cart corral. There might be some consequence to society: wind might blow a cart into another car and damage them. But to oneself there is no consequence or nearly none. This makes it a perfect tool for judging someone's character.

I always return my cart, no matter how far away from a cart corral I might be. And I'm usually pretty far because I like to park at the far end of the parking lot, not to avoid my car getting hit by bad drivers or anything, but just to force myself to walk more.

Interestingly, in Japan almost everyone returns their carts to the cart corral. Some don't, but most do. I don't think this is because Japanese people are morally better, on average, than Americans though. I think it is because they feel social pressure much much more and so they get paranoid and think everyone is looking at them and judging them if they don't promptly return their cart.

This social pressure might be what caused the actions you saw in the UK.

Honestly, I think the lack of social pressure in the US is a bad thing a lot of the time. I mean, it's good. The US had a lot of that pressure before the hippy movement turned society on its ear. It's mostly good that it's gone. But for some things, I think the result has been more negative.