Ohhhh….I just realised something (because @waitingforgarridebs mentioned Montague Street as a possible filming location).
They moved the scene when John bumps into Stamford from Piccadilly Circus in the unaired pilot (in front of the Criterion - in ACD canon they meet in the Criterion bar) to Russel Square Park for ASiP. And I never quite understood, why. Perhaps now I do:
They made John walk through a park literally around the corner of Sherlock’s old flat.
I read this as them saying, they would have met anyway. John was drawn to the place where Sherlock lived without even knowing him yet. And it just needed Cupid Stamford to point John into the right direction to find what he was looking for.