My husband is from England and I lived there for a few years. I lived in Bristol, but went to London a lot. I can't give you info about living in London, but I can give you some general info about living in the UK.
I am from Chicago and I found UK summers to be on the chilly side. The upside is you can usually get a cheap flight to somewhere in the Med.
There are many cool museums and sights in London.
There is a better railway network than there is here, so you can jump on a train and see another UK city, such as Bath and Chester...typical Yank tourist stops.
You won't have to pay VAT (Value Added Tax), if show your US passport when shopping.
Depending on where you are from in the States, you may find an English winter more mild. I did, but I am from Chicago--so consider the source.
Things are smaller there, housing, cars, yards, some roads. How much smaller depends on where you're from here.
I don't know how helpful any of this to you, since it's not about London, per se, but my experiences as an American living there. FWIW, I had a great time living there. I love the English people and I go back every chance I get, even though it means staying with my MIL.
