This is literally the most geeky thing I have ever done so, sorry, but. . .
I watched the episode again recently and was also intrigued to read about there being an extra clue that people had missed. My dad used to work at St Barts so I know the area quite well and I can say with a good degree of certainty where the roof he jumps from is:
link
It's here (where the red circle is) and therefore pretty much consistent with St Paul's being on his left. If you look at street view for that location, we can get Watson's exact view of the roof top:
link As you can see there is a low building between where Watson stands and the hospital meaning he cannot see the ground where Holmes will land, either before or as he lands. As many other people have pointed out, Sherlock is insistent that Watson stands in the correct place. The building is not easy to see in the episode because it is cut out of the shots where Watson is looking up and in the shots looking down at him you get no sense of depth. Watson has to then run round the corner to finally see Holmes' body on the ground:
link You can recognise the bus stop and the pattern on the pavement.
I don't think i've solved this for anyone, it's just the insight that I have. Feel free to disagree