You're right, Tangfastics, and I admit, I am surprised by that.
From the BBC: "There's nothing in the law that specifically bans taking photos, but the Electoral Commission very strongly discourages any photography inside a polling station, primarily because of complex laws about maintaining the secrecy of the ballot. For instance, it's illegal to reveal how someone else has voted, which could happen inadvertently via a sloppy selfie. In addition, taking a photo of a ballot paper's unique identification number is against the rules. The key is a law against releasing any information "obtained in a polling station", which is in order to protect the integrity of the poll.
The Electoral Commission says: "Due to the potential breach of the law, intentionally or not, we strongly advise against any form of photography taken inside a polling station. However, if a voter would like to highlight their participation in the elections, we suggest this is done outside the polling station before or after they vote." Consequently, you will see "no photography" signs inside many polling stations."
Mine has a 'no photography' sign, and I had assumed that was a legal requirement.