You could try, "Smile slightly at the lens, as if you two have a shared secret that you don't want to give away to the rest of the people there"? Would that work? Or maybe you could just think, "I'll be in drinking mai tais on some Caribbean island on New Year's Eve and not zooming around London trying to save more idiots from themselves while dodging other idiots who think vehicles with blue lights should give them way"? Ideally you want to present a more or less relaxed and confident look, with a post-orgasmic smile (but not -- and this is important -- a feces-eating grin).