Well, having my first solo in 1959 and left the RAF in 2003, my memory is fading so perhaps more learned pilots will correct me. But here goes: You change to 1013 at Transition Level which is normally 3000 feet QNH. In the USA, by the way, it is 18,000 feet. I think the London control zone use to be 4000 feet. I think quadrants are now gone and we operate semi-circulars.