The Queen held an audience at Buckingham Palace.
The Queen today received Letters of Credence from Thailand's Ambassador to the United Kingdom, who is wearing a black armband (and whose wife is clad in black) as part of the continued national mourning for the October death of King Bhumibol Adulyadej. I mention this because you'd be forgiven for thinking it was related to today's terrible incident near Parliament, after which the gates of Buckingham Palace were closed. (I believe the audience was completed prior to those events. Her Majesty's next scheduled event happens to be a long-planned visit to the Metropolitan Police tomorrow, to open their new headquarters building. Perhaps things will be postponed or will take on a different purpose.) UPDATE: The Queen's visit to New Scotland Yard tomorrow has been postponed.
A brooch was probably worn, though it can't be seen.