I got to visit London for a few days with some friends while traveling on a European tour with Contiki in 2010.
I found just walking around the city to be amazing. As an American, I really could see and feel the connection between the two countries. The influence was very visible. London is a gorgeous city though, and it also had a quaint type of feeling that I hadn't expected.
We split our transportation between The Tube subway and taxis. If you have more than a few people, taking a taxi may be the better deal as The Tube can be quite expensive. The system itself is old - the train cars are rather small, and you'll find some stations with uneven floors and winding tunnels. There was even one where we had to wait for cargo-sized elevators to take everyone up. They make for great "I'm in London" pics and will get you to where you want to go, but it wasn't exactly my favorite part of the city.
While we mostly stayed with stereotypically foods like fish and chips and lamb, the city is pretty diverse both food-wise and culturally. While I've heard stories of discrimination, I didn't come across anything like that and found everyone to be well-mannered.
We spend a day to check out The Stonehenge. It's definitely worth it but do keep in mind that it takes a few hours to get to by tour bus, so it'll likely take up a huge portion of the day.
More than anything, the simple feeling of just being in London was great because like I said, there's such a connection to my own home country. I definitely wouldn't mind a few more days at least to spend there, and hopefully I make it back sometime.