Back from the other side of the pond
1) If you are prone to motion sickness, and they announce that you will have to board the plane and THEN wait two hours in a stuffy and hot cabin while it's de-iced, for the love of all that's holy, buy Dramamine.
2) Bring candy with which you can make amends to the flight attendants for the state in which you left the airplane lavatory. Seriously, it looked like a passle of hungover hobos had spent the night in there. Ick.
3) Leaving most of your stomach contents behind does wonders for fighting jet lag.
4) Brits are unfailingly polite...except when it comes to the Tube. Now, even in DC, we wait for people to get off the metro car before getting on. That apparently is an unknown concept for most Londoners.
5) Don't fool yourself: if you're not much of a museum-goer under the best of circumstances, you will have zero interest in tramping around one after having gotten half an hour of sleep on the flight over. And if you are in a modern art museum, bring someone with you who you can make smart-ass comments to, a la "this is art?" Otherwise, you'll scare and annoy patrons of the museum who just want to wander the galleries in peace.
6) The best way to make a million new best friends in clubs is to have a pink feather boa. Mine, which I got in the Camden Lock as part of the club kid gear that Frequent Flyer insisted I buy, made the rounds and at one point almost got away from me.
7) If you wear pink sunglasses indoors at night, and you accidentally knock over someone's full glass of beer, there is no way to avoid looking like an asshole. This would be where your American accent comes in handy, as we all are imperial pigs who don't give a rat's ass about other people's belongings.
8) If the menu in the Indian restaurant has three chilis next to a dish, and only one next to a vindaloo dish, it will be scorchingly hot. It also will be extremely delectable.
9) You can choose not to check your coat at a club and stow it behind a pillar. Just be prepared to smell like a brewery.
10) If your boots are so worn down that they have no grip, then SLOW DOWN while sprinting to catch your flight. Otherwise, you may end up wiping out on a moving walkway. My knee looks like I took a cheese grater to it. I'm thinking of telling people a much more exciting story, like that it's road rash from my Harley.
11) Make sure that the friend you're staying with is patient and generous. Thanks, Frequent Flyer, for being such an excellent host! Hope that I didn't leave too much blue glitter and pink feathers behind.