Your question is one that I can answer, yes.
The boundary line will be the brickwork edge, because this is what stands on the land itself. The airspace doesn't define the boundary line, it's defined by physical features on the land itself.
You do often get a degree of overhang, you're right, but that doesn't alter the boundary line. It's simple an overhang. If there had been a dispute, for exmaple, a party wall surveyor would have been engaged and under the terms of the Part Wall etc. Act 1996, the surveyor would simply authorise the overhang, as is his powers under the Act.
Put simply, the boundary is by reference to the physical properties, not the overhang.
Usually, it's where things have stood for 12 years or more.