Our trip to Scotland was via London, with some time to spare before heading to Euston. So we hired Boris Bikes to explore London at night.
Buckingham Palace, which was surrounded by police, and where F thought he'd spotted a couple of friends. Parliament, where we learnt that the police presence was because of a V for Vendetta-style march and met the friends F thought he'd seen earlier (small world, or Snapchat maps).This could be a tradition.
NTS: if you're going to ask the police what's going on, try to ask the ones who aren’t all tooled up in their riot gear.