1. I bring an umbrella with me everywhere. Even if there is no rain in the forecast (0% chance of precipitation is more of a wish than a prediction), even if there are no clouds in the sky. Stealth rain attacks are common. A single, dark raincloud will appear out of nowhere, usually while you're out walking the dog. It will track you down, unleash a deluge directly on top of your head, then continue on its way. I'm serious. Bring an umbrella.
2. I simply ask "Alright?" instead of greeting people with "Hi, how are you?" It's the most common way for Brits to greet each other, even though it sounds like you're asking for permission for some unspecified thing. People then respond with the same question. "Alright?" No one ever divulges whether or not they are, in fact, alright.
3. I can enjoy a room-temperature, flat beer. It all comes down to the difference between ales and lagers. Most American beer (bottled or on tap) are lagers, best served cold and with lots of carbonation. Typical British ales are meant to be consumed at close to room temperature and have very little carbonation. At first, I found ales almost undrinkable, but I've since grown to appreciate their more complex and interesting flavors.