Julian
The Rule applies to aircraft taking-off from or landing at an aerodrome within controlled airspace , not as you refer to, within an "ATC zone" (what's that?) or just an ATZ.
If you intend ballooning from a field within a controlled airspace where this is permitted, why not contact the ATC manager at the controlling aerodrome - or if you want a ruling on whether your field is included in the legal definition, write to the CAA and put them on the spot! You could argue the case, if you so wished, that your field is not "commonly used", but then what is the spirit and intention of the Rule anyway?
2 s