In years past, I’ve covered the Electoral Commission’s verdict against allowing selfies in polling stations due to the numerous legal issues they raise, such as the ban on sharing “information obtained in a polling station”.
The Electoral Commission’s advice this time around is:
The law relating to obtaining information in polling stations and disclosing such information is complex. Given the risk that someone taking a photo inside a polling station may be in breach of the law, whether intentionally or not, our advice is against taking any photos inside polling stations.
(There are issues about the secrecy of the vote, the secrecy of the security marks on ballot papers and also on not intimidating people when they go to vote.)
Remember too that the ban on publishing exit polls before 10pm on polling day also covers sharing information about how other people have voted. That therefore includes banning sharing messages from other people saying who they have just voted for. See more details in my exit polls piece.