Here are 10 interesting differences between the UK and the US, observed by myself, an English guy living in America:
1) Religion. Most British people are very secular and religion plays no role at all in daily life. Politicians who mention religion are mocked. Few people go to Church. On the other hand, saying grace is still common across America, politicians virtually have to be Christian, and atheists are broadly looked down on.
2) Cars. In the UK cars have to go through regular checkups by law ('MOT'), so the cars on the road are in good condition. In the US however you'll commonly see cars spewing fumes and with dents all over. In the UK we drive on the left, in the US on the right. The UK generally has manual (stick shift), while the US has automatic.
3) Salt and pepper. This is a weird one you can learn the hard way. In the UK you have one hole for salt, and multiple for pepper - in the US it's the opposite.
4) Expectations. US culture is very much driven by a sense that everyone should be able to get what they want and also should have maximum freedom. It's uncompromising. On the other hand, in the UK you are expected to follow all kinds of social rules and stay in your lane, to avoid inconveniencing others. This difference is probably the biggest one as it can be seen everywhere, especially in politics and issues like the gun debate, the British monarchy, and in how politeness is perceived.
5) Conversation. In the UK you have to make eye contact with someone before talking to them. In the US people complete strangers will talk to you out of the blue while facing away from you.
6) Weather. Generally the UK is usually overcast with clouds, with temperature that is mostly comfortable enough all year round. We might get winters of around -3C (on a bad day) to summers of around 30C (on a particularly good day). In the US on the other hand you'll get winters that are -15C and summers that are 35C for weeks on end. Obviously that varies depending on where you are in the US. Also, the sun is much higher in the sky due to the latitude, so it's always a lot brighter - which makes you feel really good compared to Britain's gloominess.
7) Sizes. Everything in America is bigger, it's done on a bigger scale. This is probably down to the fact that it's necessary given the size of the country, but also possible due to the available resources. From the perspective of a British person, it's great to be able to drive anywhere and park without even having to think about it - because in the UK you have to negotiate your way around winding narrow roads and then often really struggle to find a spot, if parking is allowed at all, and often you have to pay for it.
8) Language. It goes without saying that British English and American English vary in many ways. My wife and I (who are super-nerds) keep a list of differences we've noticed, which is up to about 650 right now. Sometimes it can get really confusing. For example, a 'pillow' to a Brit belongs on a bed, but to an American it can also be on a sofa (which we call cushions). Generally to an American a cushion is something you have to sit on. So you can really get twisted around when it's not only that you use different words, but words have contradictory definitions. Another example is 'cot', which in British English is where a baby sleeps, but in American English is a type of bed an adult might sleep in.
9) Traffic intersections. Americans are really late in the game when it comes to roundabouts. They are starting to pop up across the country, but generally America has a lot of stop signs and traffic lights. This can mean driving through a downtown area can take 3 times as long as it should, due to the constant stop/start traffic.
10) Eating habits. Americans very often won't use a knife when eating, may just use their hands, and usually use a napkin. For most British people napkins have not been used in the home for 2 generations, only a few foods should be eaten with your hands, and if you use a fork with a proper meal then you'll almost always use a knife with it.
I'm not saying either country is better than the other, I just think it's really interesting how different people can be, even though we almost nowadays function as one cultural group nowadays.