OCTA can operate up to and including the base level as depicted on the map.
Showa Cho,
You quote Australia, ICAO and the rest of the world is out of step --- again.
As I said, the ICAO SARP is that the whole of the volume of "controlled airspace", right to the boundary, is available to aircraft in that airspace. For vertical buffers, ICAO quote a figure in meters, that is rounded off to 700'.
Our AIP does quote navigation tolerances for aircraft in G to ensure that lateral boundaries of controlled airspace are not infringed. Unfortunately, because of the complexity of many of our airspace boundaries, VCA are a very common occurrence.
In my experience, vertical limits are always in AMSL on charts, not AGL.
Tootle pip!!
PS: Last time I looked at the London area chart, outside the zone, the first TMA base was 1200 AGL to the SE.