A quick tourist note - if you want to dodge over-priced museum cafes, Caffè Tropea is incredibly close to the British Museum. It's a great spot to stop and recharge after a full day of sight-seeing.
If you find yourself craving a cappuccino, you may just find yourself with a chocolate frenchie on the top.
Even if it weren't for the staff, The Espresso Room has granola bars that could make one dare to brave Central London at rush hour.