So you are ready to move to the US: You've been to the states a few times, maybe longer; you've dated or are married to an American; you've studied up on US history, or read every story written last year by every newspaper in the UK about America; you're a big fan of American movies. And after all, it's the same language. Simply put--you know America, and you are ready to come over!
You are in for a big surprise.
There are tremendous cultural differences between the US and the UK, despite having many similarities. People talk different, and not just in the accent or the words, but the general tone and meaning. They have different ways of making friends, finding jobs, interacting at the office. The social system is vastly different from what you know, and religious differences may come as a bit of a shock.
And when all this comes together as part of a big move, you can find yourself wondering 'what have I done???'
In some ways, try to envision being an expat in rural Mongolia, with a different language, culture, climate, time zone--in essence, nearly everything vastly changed from what you accustom to. You go in knowing it is going to be an adventure--there will be things that don't make sense and things that seem strange. Approach your trip to the United States in a similar way, something radically different from what you expect, and you may find the transition goes a little more smoothly than those who think it is just a simple 'hop across a big lake'.
Now, as for some specifics...you could write a book, or several, on some of the differences. Without trying to get into stereotypes, a few observations:
1) Americans are outwardly very friendly, they will talk to strangers in a store, laugh with someone at a bar, and help their neighbours in a pinch. That said, many ex-pats have reported difficulty in making 'really close friends' like they had back in the UK. The friendliness is only on the surface, so many report.
2) Americans generally do not like / 'get' satire. Some ex-pats claim "they just don't understand it" while many Americans think it "is just a waste of time by those who have nothing substantive to say." Satirical comments in the workplace are almost always misunderstood, and generally not done in social settings either.
3) Many Americans are very family-oriented (at least those with families). Family life is an endless parade of school musicals, extracurricular sports, birthday parties and the like. Many new ex-pats report exhaustion trying to keep up with all the things to do with the kids when they come to the states.
4) The majority of Americans have never been abroad. They may have heard of Spain but have no clue about Ibiza for example. American media does not speak much about Europe, let alone the rest of the world, and the issues facing 'Brussels' and the 'EU' are of absolutely no interest whatsoever to 99.999% of Americans (in fact, polls indicated that a majority has negative feelings toward the UN). On the converse, the United States is a huge country. You could fit most of Europe in only 1/2 of the area of the United States. If you want a more direct comparison--the entire land mass of the United Kingdom could fit within the state of Michigan. Go look at a map of the US to get an idea. Many Americans feel they simply haven't seen enough of their own country to warrant travelling abroad to see another country. Just as Americans are made fun of for not seeing the world, many ex-pats are ridiculed for seeing only New York and Florida or other large 'tourist' cities without seeing the vast expanses in between.
To add, many people just don't get enough holiday time to make it financially worthwhile travelling abroad, with holiday - "vacation" time often being as low as 7-10 days a year, and in some instances that includes sick days, the norm being that you have worked for a year before really getting any holiday entitlement...because of that, you'll see a live to work rather than a work to live outlook on life.
5) Most Americans are 'hyphenated-Americans' and proud to boast of Scottish or Irish or German or Italian ancestry, even if it has been centuries since anyone in their family lived overseas. It is said that Europeans do genealogy to prove who they are whereas Americans do it to see who they were and what they've become. Many Americans like to remember this not because 'they like the Old Country' but they like to show that their family was once immigrants who dreamed of a better life 'in America'. By pointing out that fact (and hopefully living a good and successful life), many feel they are honouring and meeting their ancestors wishes and dreams.
6) Americans will joke about English people's teeth, for what it is worth. If you ever get into an insult match, expect some dentistry comments. Monty Python and the 'Killer Rabbit' with 'Big Gnarly Teeth' is as much to blame as any British dentist.
7) Making friends can be daunting. While for someone in their 20s they can reach out to folks who are also 'just starting out' and eventually form some lifelong bonds, for some who are older, in their 30s and 40s, with kids and a family and their own way of doing things, meeting others can be difficult. Some put it down to Americans being somewhat cold to newcomers, but others note that the older you go, the harder it becomes to make friends in nearly every culture. Don't expect that the people you meet in your first few months are going to be as close as the ones you went to school and university with back in the UK. It will take time, and as it was back in the UK, cannot be rushed.