Realistically, no-one's going to care about running a cable to your neighbour. If you start running cables to multiple neighbours, or connecting the cable directly into the mains supply of the other properties, you may attract attention.
Mostly for the potential of microgrids to upset the delicate balance of power-delivery and frequency-stability of the wider grid. There are a few initiatives around peer-to-peer power sharing and microgrids, but nothing particularly mainstream in the UK yet.
