1. Gum is not a big deal here.  Chewing gum for Americans is like smoking cigarettes for the British.
2. Tight pants are not only for the emo kids. That's right, many of London's fashionable men are pulling off the snug fit.  Also, I've noticed that American men, for the most part, seem to have smaller bums.  Maybe it's just my imagination.
3. Apparently I look like I know what I'm doing.  I've been asked for directions by British people several times!  Today a british teenage girl approached me... "Ello, do you happen to knoooowww where thuh KFCeeeee is?"
6. Kit Kats are a big deal.  Apparently they're better here too.
7. H&M in America is a dump...  compared to the H&M in the U.K.  The American stores are comparatively outdated and the clothes are plain and ill-fitting. 
9. Londoners don't wear their sunglasses at night.  Or hardly ever, really.  Maybe because of the rainy season...
8.  Tip for crossing the street: If someone else crosses the street, cross it with them!  In the U.K. people tend to dart in front of cars to cross the street.  When the first brave soul leaps onto a busy street, it's usually safe to follow close behind. 
10. Someone sneezes.  Your response: "God bless you."  Their response: *silence...no eye contact*  I've concluded that saying,"God bless you" post-sneeze isn't protocol in the U.K.
11. I've seen far more attractive women than men.  That is only my observation.  Perhaps it is because I am often in a fashion-centered part of London.  I will suspend all further judgement :)
12.  London bakeries are more like art galleries.  They have beautiful sweets and cakes.  I'm obsessed with all of the swirly ones.
13. Sketchy side streets have the cutest shops.  I found this cute shop full of scarves from India for only 5 pounds each!
14.  American cereal is cheaper here! Fancy that!  I might even fly home with a suitcase full of Frosted Flakes.
15.  Pay to pee.  That's right.  If you have to use the "loo" it's best to wait until you reach your final destination.  Public restrooms in London are NOT always free.  It might cost you twenty pence to take a twinkle.