US Southeast -> New York -> London
London driving is still necessary if you want to do anything out of town. It's super spread out and despite the Tube being reliable, it doesn't really run at night. London is so huge it's unlikely your friends will live much less than an hour away from you. The roads are tiny and civil engineering here post-dates most of the construction so it's mayhem compared to driving in the US. However, you can get the hang of it.
The NHS is great for (1) the bottom end, basic stuff with a straightforward solution like a broken arm and (2) the high-end emergency stuff like childbirth and cardiac arrest. Assuming you have reasonable health insurance in the US, the NHS falls flat for any kind of non-emergency, non-routine procedure. You'll need to pay retail to get the same care which is about 1.5x what the same procedure would cost cash in the US. Otherwise you'll learn the NHS's "holistic" solution to your problem. You will have the comfort of knowing the less well off aren't in dire straights paying for basic healthcare. If you think it's "free" though, see below.
Taxes are out of control. In addition to about a 50% income tax, most goods and services are also subject to a "value added tax" of 20%. Compensation is also about 60% of the US version. So in exchange for the NHS, you get an "everything is expensive" mentality, and it's super common to hear well-to-do folks complain about the slightest expenses. It's actually very socialist for being one of the largest capitalist democracies in the world.
The law here is much less clear, and somewhat "equity-based" although followed. For example, an individual has recently been convicted of the crime of owning the Anarchists Cookbook. Penalties are less severe.
Real estate costs are 1.5-2x those of an average US city and 1.25x those of New York. While new construction is made to last longer than its US counterparts, e.g., using a lot of brick, most of it has already lasted longer than its US counterpart. In other words, in London there isn't much new construction. Instead, it's a ton of 100+ year-old homes built with period materials and including 10, 20 and 30-year "extensions" strapped on.
The food is generally lackluster except a few categories like Indian. Forget pizza or Mexican food exists.
People are about as friendly as anywhere.
The weather is almost unbearable. It's always somewhat cloudy. There may be a few consecutive days in September that are clear, but any more than 3 and people will start complaining about how "it's boiling out". In the Summer, while it does warm up in direct sunlight, you don't get continental heating and the stable warm temperatures it brings. So you might find yourself wearing shorts out in the morning and wrapping up in a blanket later in the day. You'll note people wear lots of layers.