When people need to work on live cable they use a contraption called "a Faraday Cage". This is a metal cage which is connected to the "Live" ie non-earth side of the system, and which prevents them from grounding themself (touching earth) in any way. To suffer an electric shock, current must actually flow through the body - ie from Live to Ground (or Neutral/Negative). I have watched electricians work on live circuits and on normal mains voltages, many of them will keep one hand in their pocket. This is to make sure that they don't get a shock from one hand to the other as this will tend to cross the body in the vicinity of the heart, which can be fatal. A shock to one hand will often ony be able to reach ground via legs and shoes, and a pair of dry rubber soles will greatly cut down any risks.

Incidently any coating on a main distribution (ie pylon) wire (132,000v and above) would only be to minimise corrosion and at those volages would have no insulating effect whatsoever. On the othe hand, the distance that the lines are set apart and the size of the insulators would make it imposible for most UK forna to touch live and ground at the same time.