Think they said roads were closed around Trafalgar Square and Parliament Square on the traffic news earlier, but that would only really affect people who are mad enough to drive into the city!It will be absolutely fine up there. There are demonstrations every weekend about something. Sometimes more than one a day. They actually have to book a slot... they’re not allowed spontaneously!
If we were at home we would happily be in London with that going on. You would probably want to avoid visiting Westminster Abbey and the Houses of Parliament, but they’re easily avoided. Everywhere else will be fine.