They say the US and the UK are two nations separated by a single language. Sadly, it's more than that. Just try calling your friend in Scotland after you've had dinner in Philadelphia and you'll understand just how different things are. For a start, there is the climate. While much further north, and thus subject to very different swings in terms of daylight, it is actually very temperate, with warm air coming from the Caribbean, carried across the Atlantic by the jet stream. The British may complain about it all the time, but it rarely becomes dangerously hot or cold, and even the ugliest suburban hamlet has charms you won't find in its American counterparts.
Some things will become immediately obvious, like that light switches can never be found inside bathrooms, or that, due to the much higher voltage, electric kettles boil a lot faster than in the US. Their weird outlets may be annoying at first, but you'll quickly see how much safer they are. On the other hand, there are social cues you won't know to pick up on until someone tells you, and small things, like putting your silverware in the six o'clock position when finished eating, which you should know, and probably won't when you step off the plane. Read on for enough essentials to get you started.