Barra Fina is the place to visit if you are craving some fantastic spanish tapas and cocktails. Like many London restaurants, they don't take reservations, but you will definitely be treated with Spanish sparkling (Cava) and small appetizers (Para Pica). The restaurant has a twin Fino, a less fun and more serious copy of Barra Fina. In both places I usually order Sardines a la Piancha, Razor Clams, if they are on the menu, and Spanish Morcilla Iberica.
Dinner for one: £15–35 (depending on the amount of tapas you have ordered)