UK Electric Pole Types?
Higher the tower, the higher the transmission voltage.  All transmission and the majority of distribution is three-phase (hence multiples of three).


Largest steel pylons (275 kV and 400 kV) - Transmission - 3 or 6 cables
Smaller steel pylons (132 kV) - Distribution - 3 or 6 cables
Wooden poles (11 kV and 33 kV) - Distribution - 3 cables




Depending on load and remoteness, you also have 66kV and 6.6kV for distribution.  Finally stepped-down to 230 (single-phase) /400V (three-phase) for consumers.  There are also HV DC Links. 

Conductors are uninsulated aluminum conductors with various cores. 

9 Players provide power as illustrated by National Electricity Transmission System Performance Report 2016 – 2017.

Transmission/Distribution requires transformers to step-down the higher voltages to lower voltages.


They can be wye-wye or delta to delta, but a delta-wye converts from three-phase three-wire to three-phase four-wire.  The neutral (Ground in drawing) and each line forms three single-phase 230V circuits (light loads like lights).  Feed two single-phase circuiuts to get 400V (heavy loads like stoves/heaters).



Power companies go through great effort to balance the load (even as far as reading the obituaries).  But at some point remote homes and residential areas do not need three-phase power, so single phase (two wires) or two-phases are used. 



Three-phase voltages are phase shifted by 120°, so if the currents are the same, vector addition on currents \$ \overline {I_A} + \overline {I_B} + \overline {I_C} = 0\$.  So no neutral is need, which saves copper.