Blimey, there's a wealth of places! We all tend to head out around Russel Square area for lunch. Tonnes of little Italian cafes that do really good food and coffee which doesn't cost the earth!
Evening, food-wise I would go for Thai, there's a great Thai place on Museum St (Thai Garden or something) or there's another amazing Thai place between Gower St and Goodge St tube. Or there's a few cheap n cheerful places down Charlotte St. Of course, you may not be looking for cheap n cheerful, but it pays to in London in my experience! However, having said that, never, NEVER buy the hotdogs from street vendors. My partner did (he was drunk, but it still serves him right...) and he was in bed with food poisoning for 4days. It was horrendous. The first thing the pharmasist said to me when I asked her advice was, "was it a hotdog by any chance?!" You have been warned!