I'm no expert in the UKs domestic energy, but at least looking at electricity mix charts it seems to me like growth of renewables after introduction (lets say 2005) from <5% to ~40% compensates the drop in electricity produced from coal (~33% to <2%) quite nicely. The huge drop in coal production starting ~2014 also doesn't seem to correspond to an increase in net electricity import, so I don't see what you're basing your claims on.

It's also news to me that the UK was ever self sufficient for energy in the last few decades. Most countries are at least roughly matching their electricity production, but almost all are huge energy importers for non-electric energy, so I don't quite see the issue (if e.g. most large oil producing countries were to suddenly stop exports, most of the world would be in huge trouble).

Why do you claim that this, which has been the reality for almost all countries over the last few decades, would be horrific?