Well then. The Royal Marines and part of the Home Fleet stand ready to steam up the Tames. A couple of 15"ers pointed at Parliament should convince Halifax to vacate his post.
Here's another idea for you to consider, trekaddict:
Place Halifax in an open car, have the car move slowly through London, and drive him past a sniper. Then the government can claim it was a lone gunman. However, before the sniper can be put on trial, he gets conviently killed by some random person.
I call this idea "JFK in London".