As Skylion says, the northern area was the original terminal area. As work in the central area progressed, short-haul flights were gradually transferred there during the fifties, but the long-haul Stratocruisers and Super Constellations remained on the north side for several more years.
In those days the London Transport bus service did not venture through the tunnel into the central area, so spectators had a choice of walking through the pedestrian tunnel (now used as an additional lane for cars) to use the viewing gallery atop the Queen's Building, or using the more primitive ground-level area just north of 28R. The latter involved walking across the active runway when directed by the duty policeman. There was always plenty so see and do, although the pace of life was a lot slower then. There were even a couple of Dragon Rapide biplanes available for fifteen-minute joy-rides around the local area - imagine that in today's environment!!!
Even after all long-haul flights transferred to what became Terminal 3, there were occasional visiting aircraft parked on the north apron (now under the public perimeter road).