Electricity, gas, telecoms, water, rail (and post) are alike in that there is only one trunk network, and in a lot of cases only one local distributor. Even the firms pretending to compete with Royal Mail have thrown in the towel, apart from the ones collecting pre-sorted mail and dumping the delivery on the post office.
The pretend "market" in gas and electricity supply is a fraud because the various grids' prices are just passed on. The water companies make their regulator a laughing stock.
The only competition is between gas from the north sea and gas imported from the middle east, but I recognise that there are a lot more sources of electricity nowadays. I still think that the pretence of competing suppliers costs us more than a managed supply to a National Gas board would, though.
I support nationalising natural monopolies but Labour choice of industries to nationalise is motivated by internal politics (i.e. which has the most union members) and the public by what was nationalised in the good old days... Gas is a natural monopoly and electricity transmission is but its production is not. Electricity generation is moving progressively towards microgeneration and exchanging electricity with Europe to balance out variation in supply and demand. A nationalised electricity generator would be competing with every person with a solar panel on their roof and every community hydro project etc. Post is not a natural monopoly either. The universal letter service is surviving due to junk mail and there are companies that provide door to door delivery of parcels without any involvement by Royal Mail. No one would seriously consider nationalising Royal Mail if did not have a very large union membership and did not play into peoples desire to return to a mythical golden age.
There is some sense in nationalising open reach but why BT retail? Socialist ideology and nostalgia?
The left is blind to how deep Thatchers market policies have altered society and how difficult it will be for them to alter it. 70s socialism will be ripped apart today and policies for a 21st century socialism are only at an embryonic stage. Big heavily unionised companies are not suited to competition and technology makes it harder to force people to use one company. A new BR would be competing against a larger number of airlines than 25 years ago, against coach companies and against ride share apps like BlaBla Car and app based taxis services e.g. Uber. Every attempt to force people to use nationalised companies will just encourage attempts to break through economic controls elsewhere. To force people to use a nationalised BT it would have to take over the 4 physical mobile networks, Vodafones business broadband network, Virgin Media and every satellite broadband company opperating in the UK...