National Grid have used helicopters for live work in the past.
BBC - National Grid linesmen practise repairs from helicopter
Google doesn't find many recent mentions though. Most mentions relate to helicopters using infra red cameras and sensors to inspect lines.
The scaffolding and nets are not there to protect the workers working the on the lines otherwise they would cover the full length of the line, not just roads. The purpose is to make it easier to raise/lower cables while installing new lines/refurbishing existing lines. They also protect road users if the line unexpectedly drops during the work.