I like to get out of the house on weekends and do work somewhere I can get a good Coffee and free wifi. These are my favourites in the little place I live called East Finchley.
Campbell & Syme
Campbell & Syme win it for me mainly due to their incredible coffee. The place is run by two friendly chaps who really know their stuff when it comes to quality coffee, so you know you're always going to get a decent single source cup. The wifi is fast and they don't seem to mind you staying for a few hours.
The Phoenix Cinema
The Phoenix is a really great place to chill for hours on end, due to the fact that it's usually empty except right before a film starts. The selection of herbal teas here is awesome, and they only cost £1.30. Only drawback is that it doesn't open until 11am, so no early morning working.
Dan & Decarlo
This place is in a decent location and has the best opening times, 7am - 7pm, and there's almost always a table. The coffee here isn't brilliant, but it's the cheapest and you get a massive cup for the price. Grab the wifi password because the 'free' BtOpenzone wifi sucks.