... If you stand at the North end of the platforms at St Pancras you can see a neutral section sign just inside the Canal tunnels. Obviously the section needs to be broken up somewhere between Finsbury Park and St Pancras but it will be interesting if a 12 car ever has to stop with its back few coaches in the platform there (passcom, egress etc).
The class700s are powered as two separate 4 or 6-car EMUs with arrangements for dual powering of essential support systems. So if you are implying that a train halting over the break between the MML and ECML supply networks with both pantographs up (as is normal) will cause issues, I'm sure that has already been thought of and catered for. There would be no chance of one side connecting to the other. In fact that is more likely to be the saviour of a breakdown on one supply as the feed that was operational would be available for the train to self-recover. The class 700s have more than enough tractive effort to ascend the gradients in the core with half of the traction equipment disabled.