Hello, I’m an American going to London. What do I need to know about the UK?

So I’m just visiting for a few days and I’m curious about how the country works. What should I know that would make my trip more enjoyable? I’ve been to several states and, just to let you know, I’ve noticed some differences in places, like where things are put in the grocery stores and what brands are available. What are the differences between here and the US?