You just need an adaptor or two.
Check what is more common in the area. I used my card almost everywhere in Europe, but in some places it's kinda faux pas, especially for smaller purchases. I'd carry the card and bring about 20-50 in cash everywhere, depending on what you're doing (like whether or not you're going to a bar or something where you'll be making a lot of small purchases).
Don't bring food, they're pretty strict on food. You'll find better food there anyway.
Go anywhere you want, man. Traveling through Europe is pretty cheap and it's easy to find flights to almost anywhere. Just research a little bit and figure out what type of place fits what you're into. I really enjoyed Prague as a nice, cheap, fun place though. Just go wherever fits your schedule and budget.
Just relax and have a good time. I've never been to London but I've heard good things, aside from it being a little pricey. Do as much as you can everyday.
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