Beep Boop BEEP BOOP I AM A ROBOT! XD
My current Saturday job is in retail, and I don't mind it.
It pays decently, and its easy work you can mostly go autopilot on if you don't feel up for a conversation.
It helps one to develop their small talk, which to some people, (like myself), doesn't come naturally.
Plus I love seeing cute little kiddies. They love it when I goof off, and the parents don't mind either.
I tend to be able to diffuse most problems by genuinely getting worried for a customer, even if there is absolutely nothing I can do to help them, (so sometimes they are like "its not your fault, its X's", or they laugh along when I make some witty meta-humour about the whole situation).
I guess in the UK, most people are quite polite in these situations because its relateable and seen as pretty childish to rant over something trivial.
I rarely get complaints and a person like me (introverted, not normally smiley unless I'm in a good mood), normally is perfect fodder for some grumpy old bag to moan about.
I think the trick on checkouts is to try to be aware of the shoppers ability and adapt yourself to it. ("Be water my friend"... XD)
I hate it myself when I see people just launch stuff at people, and I make extra effort to keep to a customer's pace, and to try not to overload them with stuff, or just wait if they need to keep up.
In the end of the day, you can only go as fast as the person you serve.
EDIT: Good tip for checkouts - Always try to pre-prepare bags if you get some time to. Handy in a pinch.
But when you get really good at the bag opening technique (wet fingers, slide the middle, pull and twist), you can open like 3 or 4 bags before most people can even do one, and if you can keep aware of how many bags there are, you can keep an almost constant and consistent flow.
(Which I think customers really like. Showing that attentive awareness, getting bags they need without even having to ask)