The Null Device
Apparently, London is experiencing somewhat of a coffee renaissance, with a number of good cafés having opened over the past two years or so, many of them run by Australians or New Zealanders. Time Out has a helpful map of notable cafés in London here.
Having said this, London's still the sort of city where one has to go out of one's way to look for drinkable espresso; whereas in Melbourne or Sydney (or, by all accounts, in New Zealand), the baseline of what a café can offer without going out of business is fairly high, here one takes one chances. (I still remember the cup of bitter, watery bilge and overboiled milk that passed for a cappucino according to the haircut behind the bar in Hoxton who made it.)
Anyway, it appears that, like most interesting activity in London, most of the action is happening around the East End, with places like Climpson, Prufrock, Taste Of Bitter Love and Tina, We Salute You (which, it seems, is within walking distance of my previous residence) getting mentioned.