publications.parliament.uk/pa/ld201719/ldselect/ldeucom/63/6308.htm
Some stuff about the Internal Energy Market (IEM) and market coupling.
Pretty dry, but basically sharing electricity through interconnectors really, really does depend upon agreements being in place. It is not up EDF France (or any other supplier) and they don't physically control the interconnectors anyway, even if they were mad and maverick enough to start operating outside of the law.
"The Internal Energy Market (IEM) in gas and electricity has a legal basis under Article 194 TFEU. It was designed to increase efficiency by introducing competitive forces into energy markets across the EU, thereby reducing prices and improving services to consumers; and to achieve greater interconnection of markets, which would reduce the need for reserve generation capacity, thus further reducing costs.
Members of the IEM include EU Member States, plus Norway, Iceland and Liechtenstein as parties to the European Economic Area (EEA) agreement. There are no participants from outside the EEA...."
"....55.Witnesses highlighted a number of particularly valuable features of the IEM. One of these was market coupling, by which IEM participants use a shared algorithm to arrange cross-border electricity trades. The Aldersgate Group told us: “Market coupling is currently estimated to be worth £100m/year to the UK because, through interconnectors, it provides trading efficiencies which match supply and demand efficiently and thus lower intermarket and transaction costs.”89 Energy UK argued that GB operators could be excluded from market coupling post-Brexit, “as there are no provisions in the texts for ‘third countries’”.90 This was borne out by His Excellency Jean-Christophe Füeg, Head of International Energy Affairs at the Swiss Federal Office of Energy, who told us that Switzerland was excluded from market coupling,91 and as a result “trade between Switzerland and the rest of Europe is handled in a suboptimal way"
Note the above was suggesting it might not be possible to continue efficient cross-border energy sharing even in the event of an orderly Brexit... let alone one where no deals were agreed at all!