The companies I've found so far are:
The Photobooth Co.
I've managed to find a local photographer, Amy Wass, who will do a booth for a very reasonable £295 which includes a coffee-table photo guestbook, but she sets up a mini-studio rather than a booth. It still sounds fun but it does take up more space at the venue and I really liked the idea of black-and-white strip photos stuck in a hand-written book. I think the anonymity of a proper booth would lead to sillier, more fun photos from my guests.
Does anyone have any companies to add to this list?