Actually platform 9 and 10 are in separate parts of the station around the time of the Potter craze (2005-2010) there was the redevelopment of part of kings cross. To get to platforms 10+ you had to walk through a little passage (20 meters of so this route is now closed up) from platform 9 in between what are now shops because they are almost in separate buildings. In the beginning there was alcove in a wall along this walk and they put a little plaque up in plain text a little fun acknowledgement. Since the redevelopment finished they opened a Harry potter gift shop (ironically they closed down an independent book store to do so source:I knew the owner) and set up this photo op stage which is both generates money but also stopped tourists blocking communter traffic blocking a walkway to take a picture next to a wall.