I will be in London for most of just a single day plus one evening & overnight stay, and plan to find some fun things to do as a tourist. I also plan to buy some souvenirs, sample some food, and so on.

Can I rely on credit cards in most places, or do I need to get some currency? If so, is it better to exchange in advance before I arrive (in the USA), or can I wait until in the country? Where in the country can I find a more reasonable exchange rate—I suspect Heathrow might not be the best place to do that.