The Mini Tour

I'm going to Oxford and London for a short overseas vacation. Since this is my ex-Americas trip, I would be Most Interested in advice and common pitfalls I should try to avoid. For example, what is different about US international departures versus US domestic departures? Is this "arrive three hours before your flight" advice in deadly earnest, and should procrastinators of the world budget their time accordingly? What can I expect at immigration at the other end? I'm bringing one carry-on, and my passport is good for more than six months out. Also, how bad should I expect the US customs experience to be on the return trip?

If one of these pitfalls is "don't book the bare-bones airline", sadly, it's too late. The money saved on airfare is being rolled into noise-cancelling headphones (not earbuds).