That goes for the U.K. too. We have single units for 3-phase delta-wye MV-LV xfmrs, like this pad-mount type:
or this pole-mount unit:
(Both of these are 11kV delta primary, 240/415V wye secondary.)
Single-phase transformers are normally used only where a single-phase 240V service is required for one or two isolated houses.
Re "distributed neutral" -- I'm refering to the situation where MV distribution is 3-phase 4-wire, so that a neutral is run along with the phases, thus enabling a 3-ph xfmr to be connected either delta or Y on the primary (and single-phase xfmrs to be connected either between two hot phases or from phase to neutral).
In my travels, it is only in the U.S.A. where I've seen a distributed neutral on MV lines. Admittedly my travels on this side of the Big Pond are restricted to the U.K., France, Ireland, and Spain.
Could it be that your rules are just specifying a wye system on the LV secondary side? That seems much more likely.
Delta LV supplies seem to have been a particularly American system.
[This message has been edited by pauluk (edited 08-06-2003).]