In London: I stayed at the Walrus Hostel downtown, you can walk across both bridges from there.
London felt like a cleaner version of New York to me. The ethnic food (especially Indian) is surprisingly abundant. In fact, I recommend sticking to it as English food is not quite the best in the world (save your taste buds for the delicious French cuisine to come). Tea time is all the time, pubs are crowded starting at 4pm. Dedicate a lot of time to pub time. Grab kebabs after, or else. There’s a cool jazz scene also! London has unique taxis, a really nice subway, some people like the touristy double decker bus thing (I don’t). Spend a day getting lost by the Brigham castle. I didn’t do Hyde park based on time, but it was recommended. If you walk from the bridge past the parliament (the one next to Big Ben, note: try Ben’s Cookies) you can get to Buckingham palace via St. James park. That was one of my favorite areas. One night, we had dinner at a place called Wagamama then drinks at a Mexican bar/restaurant with a deck overlooking the river (sorry, I’ve forgotten the name but it will be right across the walkway from Wagamama -you’d see it for sure). It was cool because they light up the London Eye once it’s dark & it looks pretty reflecting on the river etc. (if you want you can ride it, but it’s SO SLOW). In hindsight, I would have gone to see a soccer match.