The electricity thing is a very interesting one...
As I understand it (and am no expert but very interested in energy supply, particularly how the transition to renewables is managed) its not so much that electricity has tarrifs, but that the sharing of it through the interconnectors is made possible by us being signed up to a mutual agreement with other EU countries. It should be possible (I think) to still do this after Brexit, but not in a no deal scenario: We are about to leave the current agreement (along with a whole host of other ones) and need something to replace it for things to carry on as normal.
"No Deal" means we don't have a replacement deal.
Balancing the national grid is very complicated. It always has been, but is even more so now as not only does demand vary throughout the day/night, but the supply available does too (renewables have priority access, but when the sun isn't shining and the wind isn't blowing extra capacity has to found from somewhere else).
So the mix of energy types feeding into our grid varies.
It is not as simple as "turning up" a power plant at peak demand, and "turning it down" at low demand. Energy sharing allows us to export electricity when we have a surplus, and import it when someone else has a surplus and we don't. Its a really clever way to smooth out the peaks of demand and supply, and makes it possible to balance the grid.
This is an interesting website which shows where your electricity is coming from:
gridwatch.co.uk/
At the moment we are importing 4.6% of our electricity from France, 2.4% from Holland, and we are also exporting some to Ireland. This will change throughout the day. The way the interconnectors are controlled relies on some very complex rules to make it fair for all sides.
If we leave with No Deal on electricity sharing, then I can't see how it would be possible to keep doing this (at least in the short term).
So balancing the grid will be harder. On cold, dark evenings of peak supply there will have to be another way of balancing the grid. There are (expensive) agreements in place with some companies, who agree to shut down work when supply is low. There is also some extra capacity available from (very environmentally unfriendly) generators. But if this is't enough, we will be looking at Brownouts or blackouts at least in the short term.
In the long term, if we want to move towards more renewables in the mix then an increased cooperation with neighbouring countries would make this aim much more achievable. Very interesting to look at how this could work with offshore wind farms etc. Unfortuately despite the clear benefits, I worry that there may not be much political appetite for "European Super Grids" (or European Super" anything) in the current climate.