I live in Los Angeles where, you might imagine, we have movie screenings with cast and/or crew Q&As on a fairly regular basis at some movie theatres. The last two I attended were Back To The Future, and Ghostbusters. At both screenings, there were a handful of costumed fans in attendance. At the Ghostbusters screening, they even invited all of the costumed fans up to the stage for a picture with the guests.
Of course, Los Angeles is not London. I don't know what the local customs are. I did go to one of the Harry Potter movies on opening day in London, and there were a lot of costumed fans. Probably as many as you'd see here, if not more. So I would venture a guess that costumed fans at a Q&A screening would be acceptable on a similar level.
And if it's at a fan convention, then convention customs apply.
On the other hand, if it's at a trade show or professional convention, then cosplay is probably discouraged as unprofessional.