Throw a Dyce - NZ TMA's are generally out to about 40NM from the Major Airport in the TMA and up to around FL250 (there are variations). The airspace in the TMA's aren't sectorised as such, but are shared between 1, 2 or 3 radar controllers (depending on the location). All controlled airspace in the area of responsibility is available to the controllers and they even look after IFR outside controlled airspace within the lateral dimensions.
10W - I don't know of any issues NZ had with level busts due to altimeter settings being done late. NZ also gets QNH values around 980hPa, the TA/TL used to be 11000ft/FL130 and most Area/TMA standard descents are 10-11000ft - if it was going to happen it would have been on my watch
5milesbaby - What about the current situation? From what you have indicated, I'm guessing the problem of vertical split sectors is probably an issue around the current TA/TL as well. Wouldn't moving the TA/TL only be moving that problem, not creating a new one??? But as I don't know my way around the London TMA, I'll bow to your inside knowledge.