'And this is connected with Brexit how, exactly?'
We aren't self sufficient in Electricity and presently we 'enjoy' a simplified relationship with other EU countries via interconnectors – between the UK and Ireland, France and the Netherlands which have a capacity of 4GW, and capacity to provide 6% of Britain’s power supplies. There are also 11 new connections linking the UK and other European countries either under construction or planned, meaning imports could provide more than a fifth of the country’s electricity needs by 2025.
Trade barriers (especially under a No Deal scenario)could increase the cost of importing gas and electricity across the Channel and there will almost certainly be a cut in European investment in the UK’s infrastructure.
We will no longer be a member of the EU Emission Trading System (EU ETS), which puts a price on carbon through trading of emission permits.
I have a feeling that the Japanese may have got a sniff of the fact that we (I believe) do not have a replacement for Euratom, the EU nuclear cooperation treaty which we will need, post Brexit.
The Devil is in the detail.