Generation probably not, the investment required as we move to a green economy and the costs of decommissioning old generation is probably best with the private sector. The cost of gas and electricity at the point of generation is subject to wholesale prices on international markets and the only way fully reduce exposure to it is to have a mix of renewables and nuclear in the UK.
What we should be looking to renationalise is distribution which accounts for over 25% of the bill. That's the national grid and DNOs (Electricity North West, Northern Power Grid etc). There is still investment needed but much less and on the whole is money for old rope (or wires and transformers) in this case. You also take money back from the private sector as you charge businesses for new connections to the network.