It appears I have been remiss in my research in one crucial area: how the heck did my heroes get around London? I have them universally using a hansom carriage. Turns out the hansom was the sports car of its day. Two-wheeled, it used one horse, driver seated behind, and only seated two passengers. When Miss Harkness accompanies my two gentlemen, especially when in feminine attire, they would have taken a Clarence, commonly called a Growler, for the sound its wheels made as it went over cobblestones.
Below is an informative article with pictures. Enjoy!
Images of the two major types of cab are in the Gallery.