I don't have any idea when the distribution network started, so I can't help you with that.
The construction has remained pretty much the same since they started building OH lines, with changes to the designs of cross-arms over time.
The voltages varied over the country for a long time, and the HV OH has operated at 3.3kV, 6.6kV, 11kV, 20kV and 33kV at distribution voltages.
Most RECs standardised on 11kV and 33kV but we still operate at 3.3kV in some areas, and NEDL (North East) still have some network operating at 20kV, but I don't know if this includes any OH.
Cheers
Ian
Edited to correct my voltage inaccuracies!
[This message has been edited by ianh (edited 01-10-2006).]
