Before I say anything else, I'm not an Intellectual Property lawyer, so I'm probably talking complete crap, but...
It's the Copyright, Designs and Patents Act 1988. The whole area of having copyright in your image is a bit new and uncertain for me, but assuming the Act actually does cater for it (and I'm not sure it does), then I'm afraid to say that unless you specifically say 'I do not wish my image to be used' then they can use it. You can waive your rights in most things simply by not objecting to a course of action.
Personally I think the whole thing is pointless anyway, given that there's no court on earth that'll uphold a complaint for breach of copyright simply because you happen to have been captured in a crowd scene.
Might be different if they linger on you, comment on you, make you a distinct part of the documentary, but frankly I don't think you'd have much of a case even then. There are plenty of 'fair dealing' exceptions which would probably get the filmmaker of the hook, such as reporting current events and so on.
No doubt there will be other lawyers out there who will now delight in proving me wrong....
