Tate modern members bar has good sandwiches, salads and excellent cake (pictured), particularly carrot. Also does strong coffee and has a fantastic view, seats outside when the weather is better. They open late on Fridays and maybe saturdays (check) and is the perfect place for a glass of wine after work overlooking the city and facing St. Pauls. You have to be a member (my membership has currently lapsed) which isn't cheap ( about £60 a year) but you also get free entrance to all of the exhibitions and private views (for an extra 20 quid).
Tate modern also does art.