This pylon is on the flood plain of the River Thames at Kennington, an easy walk down the river from the centre of Oxford. Note the massive insulators that keep the wires from the pylons. I'd say that at all times, the wires are about 2 m from the pylon even when the wire loops down around the insulators. If the wire are at 425,000V, then the field strength is 212500 Volts per metre. The breakdown field for air is about 3 million Volts per metre, so it is well below the level that would give sparks. Calculations suggest that the insulators would need to be about 15cm long before you would get electrical breakdown.
