It's always a moving target, finding it and currently, off the grid. But just a couple of weeks ago and probably within this month - so stay at it.
All Events in Washington, D.C. | Goldstar This is where the hop on - hop off bus is frequently offered (not currently, but keep checking, they ran within the last two weeks)
http://www.travelzoo.com/local-deals/Washington-DC/Entertainment/35198 As you can see, this time the tour bus offer is sold out - but, similarly, they come back
http://www.travelzoo.com/local-deals/Washington-DC/deals/p3 just deals in general
I'm on these the above 3 or 4 times a week checking for performances, if I see anything before I go to London on May 16, I'll let you know.
Here are two other 'deal' sites
And here's the actual full fare info ($46)