San Marino Lounge and Ritzy do good latté but for consistently good cheap white coffee I'd go to one of the Colombian cafés in Brixton Market. The one just inside Granville Arcade is very small, does delicious and filling food (about as good as most of what I ate in South America this summer) and coffee costs 80 pence a cup. The one problem is that it's not somewhere to go if you are in a rush, because they don't, but are always friendly and like you to enjoy your food.
Even cheaper at 70 pence a cup is the Colombian café on the right hand side two market rows up, good food again and I'd say that's quicker to grab a decent coffee too.