Those who know London: cocktails

We are travelling down from Ipswich to see a late afternoon gig on Oxford Street next Friday afternoon. My boyfriend has said, rather whimsically, that he quite fancy's going for cocktails before we catch our returning train in the evening. Despite living in London for a year (before dropping off my course) I have absolutely no idea where to go. So, can anyone recommend an informal, chilled cocktail bar in London? Thank you!