No, it's really not THAT different. Culturally, there are a lot of differences, but nothing major. I'm a pretty easy-going person, and adjust well to new situations, and I had no problems adjusting to life in the UK. I prefer the UK to the US, and wouldn't want to move back to the US. The only complaints I've heard from American friends about the UK is that they're far away from their families. I don't think a single one has complained about the culture.
I had been in the UK for 3 years without going back to the US, and when I did finally go back for 2 weeks, it was a culture shock. So, I suppose there are differences, but as for adjusting to life in the UK, it should be very easy. You may find yourself confused in the grocery store, but after a while, you'll figure things out. On a grocery shopping trip, I'd usually have to drive down to a large Sainsbury's, stop at Waitrose on the way back into town, and then walk over to the Mexican market to get everything I needed. If you're short on time, places like M&S will make dinner easy. You can get fantastic, fresh ready meals there that take minutes to cook. I like fresh, healthy, vegetarian food, and my husband likes fatty, meaty, processed food, so it's hard to cook a meal to please us both, and getting two one-person meals from M&S was always easy and went over well with him.
One thing that is annoying is that there isn't a one stop shop like there is in the US. Asda isn't anything like Walmart. You can buy groceries and maybe a few small home items there, but you can't get absolutely everything you'd need under one roof. You may have to go to several stores to find everything. However, the grocery stores are very good, and produce is usually fantastic. It's a small island with a huge population, so everything you need is grown there or shipped in; because it's so small and things can be shipped from the ports in the South to anywhere in the country in less than a day, everything is fresh.
If you're annoyed by US politics like me, you'll appreciate that the UK is the complete opposite of the US when it comes to politics. Elections are quick and painless, not a constant pain in the ars*, and no matter who's in power, everyone hates them. It's not half of the country against the other half at all times, like in the US. Another thing I appreciate is that religion is present, but doesn't dominate society. Religious holidays are celebrated and are even public holidays, but they're far more secular than in the US.
What's really great is the clothes shopping. You can get clothes that are much more on trend than you'll find in the US, and they're not too expensive. If you shop at Primark, they're ridiculously cheap. They may fall apart after a season, but a dress will cost you £8, and a pair of shoes, £4. I find that styles in stores in the US are a year behind what you find in the UK. When I was in the US, I'd see something in a runway show that I wanted, and it took a year to get anything similar in stores (unless you have money for the high end designer clothes, obviously). In the UK, you'll be able to get similar styles to the runway that same season.
The absolute best thing about the UK is cheap flights. You can get deals to many destinations in Europe and North Africa for as little as £15 return (not including baggage) on EasyJet and Ryanair. It's not like in the US, where a round trip ticket from a major city to another major city is $300+. I flew from Edinburgh to Brussels to visit a friend for £14 return on Ryanair. I wasn't picky about the dates, so I could pick the cheapest possible flights. It cost £30 to take a bag with me, since I had more than I could carry-on, but £44 is still a great deal. To go to Asia, it's only around £500 per person for return flights. From the US to Asia, it cost me $1800. That's why the British travel far more than Americans - it's actually affordable!