Well, you have certainly crammed in all the highlights of London, with an emphasis on history. I gather you're not so interested in art. I would never travel at this pace, but to each, his own....

I agree with highflyer. Spend some time in London's parks (weather permitting). They are a feature of the city, one of the things that makes it unique, and at their best in May.

Also we always like to go to some plays. Exactly which plays depends on your taste, but a Shakespeare at the Globe Theatre is a unique experience. Lacking a play, the Globe offers a tour.

The Ceremony of Keys was low-key and fun for us because of the humorous Beefeater Alan who escorted us. Might want to request tickets 6 weeks or so in advance.