Where else to start than Platform 9 3/4s. In the first book, Harry is told he has to run through the brick wall between platforms 9 and 10 in order to emerge at the platform where the train to Hogwarts will depart. King's Cross Station have installed their own monument to this part of the story in the form of a Platform sign and half a trolley emerging from the wall. It is in the main part of the station so you can take a picture or two and pretend for a minute that you are starting your journey to the magical castle.
Since I visited last year, they have also added a trunk and cage on the trolley!
Beware of queues!