My understanding is that the zones are based on the requirements of the instrument traffic within the zone, and not the visual traffic outside the zone.
The only exception to this in the UK which I know of is the Manchester low-level route, which has little corners cut out of the edge of both the bottom of the Manchester zone and the top of the Liverpool zone, to allow entry/exit to the zone by following the railway line from Crew or the M6 from the north. (However, all the relevant documents are quick to point out that the corridor itself is not aligned with any specific line feature.)
As Koli says, many zones have straight lines parallel with the runways to mark their edges. Compare this to America, where most zones seem to be circular - presumably enabling the use of DME for finding the edges of the zones? (And in fact, a quick look at a French chart shows that at least the lowest levels of their controlled airspace - what we would call the CTR - are circular.)
Good subject!
FFF
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