Bean there, done that.
Kaffeine
One of the great Aussie/Kiwi-influenced cafés in London (see also: Flat White), Kaffeine takes its coffee seriously. For example, its baristas must have at least three years of training.
Kaffeine has two locations, both based near Oxford Circus.
Kaffeine
Climpson and Sons
Where better to drink coffee, than at a café that roasts its own beans? Climpson and Sons have nine blends, which they roast nearby. They use a selection in the café, but customers can always request certain blends they'd like to try. You can also buy their blends online.
The café is based on Broadway Market.
Climpson and Sons