The De Beauvoir Deli in De Beauvoir Town, which falls between the boundaries of Islington/Hackney.
I've always found a favorite cafe to anchor myself to wherever I've lived in London. It's probably a comfort thing. I always felt it was important to have a "go-to" place for lazy Sunday breakfasts or fresh bread and olives when you're hosting unexpected guests. In Maida Vale, we had a favorite Italian deli that sold the best parma ham and bread. We were also fans of the pancakes and hot breakfasts at the American-owned and run Plan 9. In Angel, we rolled out of bed and straight into the ramshackle, mom 'n pop, Rheidol Cafe (which was hardly ever opened, it seemed) and in Dalston, it's The De Beauvoir Deli that's been a firm favorite for lunch and afternoon cake, along with Reilly Rocket for breakfast and brunch.
Although the deli can become quite hectic and overcrowded (and service sometimes slow as a result - I once waited 20 minutes for my sandwich to be made, but it was worth it!), if you hit it at the right time and can bag a seat in the corner, then you're set for an hour or two. John and I are huge fans of their roast beef and horseradish sandwich, made on freshly baked, soft brown bread, although when I went last weekend, I also fell in love with the minestrone soup.
Did I mention that it's a great place for last-minute gifts? Perfect for say, if you're running late to a dinner party, or if you're looking for a present for a foodie. I noticed that they sell Prestat drinking chocolate and truffles, along with some very unique chutneys and sauces. Last year, I bought my brother a selection of hot sauces, since he's obsessed with the condiment (especially the kind at Nando's).
Their cakes are always impossible to resist: above is a slice of their gluten-free lemon cake, which, despite sharing with John, I managed to polish off in no time.
If you're in the area, I'd suggest stopping by.