I replied to Andee on the other thread but to recap...
If you are an American citicen you don't need to fill out a landing card. All you have to do is show your passport and a customs form to gain entry. Thats it.
If the confusion was over the customs form I alway put US citizen and residence as UK. (as Carrie stated).
Andee -- in your case I would put US as residence and list the address of wherever you're staying only because you won't have legal standing in the Uk until you get the visa and move there.