So this was one of my first theory's, Sherlock Holmes chose to meet on top of this building for two reasons.
The first, cause It was the work place of one that I have mentioned and bet you won't be surprised when I say Molly Hooper.
The second, was cause of the building in front of it, The Ambulance Station, and the close proximity it was to St. Barts.
You see here is How He Did It.
As John Watson was watching him, he was signaling to some homeless people who have been helping him with cases, they set up a stunt mattress on the building there and then when he jumped he landed in it.
Molly Hooper had a body, with a wig on that they threw off the top of the building as John was running forwards, but them he got hit by the bike.
When this happened it gave Sherlock enough time to get down and fix the scene, that included putting a fake mask on the dead man so that he looked like Sherlock, after a quick review he just walked away, giving his coat to the dead body and taking on the look of one of the nurses around, he could leave with out drawing attention.
I think this one is not as likely, but just as possible as any.
