Hi @Lizzie, thanks for that.
That's probably true in a lot of cases, but I know for a fact that the photography service IS available in London.
Anyway, I spoke to Airbnb about it today and was told that it was just a technical glitch and that I am entitled to my photosession. They said if I had problems when I got around to booking it (I don't actually want it straight away), to contact them directly.
For anyone else who is unsure, it is worth calling CS to ask, because even the reward is not showing up as an option for you on the website, this might just be a technical error and it might still be available.