Of course, nothing has changed since March 2020 that might put the public finances in a different spot.
Just nationalising the energy retailers, which may cost relatively very little, wouldn't solve the problem. Energy suppliers are charging incredibly high prices right now, so either you have the Government nationalise those which would cost many tens of billions (which just pushes the problem down the line really), or you buy the energy at the high price being charged, then sell it to the end user at a lower price, absorbing the cost somehow. Forcing retailers to absorb that cost would almost certainly bankrupt them, which would force nationalisation anyway (at least temporarily) and make the Government absorb that cost. Of course the Government is better placed to do that, but it's gonna be incredibly costly anyhow.
Gordon Brown's proposal has a lot of merit to it, if nothing else it's actually grappling with the scale of the crisis facing it. Hopefully Labour at least acknowledge what's coming down the line and don't just piss about with proposals that'll cut a few quid off bills. But really, throwing around that £2.8bn figure as all it'll cost to solve this is deeply misleading.