I work in marketing in London, and it is absolutely the norm to drink alcohol at lunchtimes if you so choose, nobody would bat an eyelid. This applies both to colleagues meeting for a social lunch, or clients for a more formal occasions (one Russian client in particular loves a lunchtime G&T and grumbles if you don't want to join her). I've been at interviews in fact, where the interviewer has had a drink, and in one festive team meeting last December, mulled wine was served to all at 11am!
