People who know London: cocktails

We are travelling down from Ipswich to see a gig on Oxford Street next Friday afternoon. My boyfriend says he quite fancy's going to a cocktail bar before catching the train home in the evening, but although I studied in the capital for a year (before dropping off my course), I have absolutely no idea where we could go.. Does anyone know of an informal, laid back cocktail bar? Informal's important - we're both a tad scruffy, you see.