I do not think that electrification of our rail network will happen. A lot of our tunnels and bridges just have the bare minimum clearance for the current stock. To get over head catenary throughout would probably need a complete rebuild of these. That effects the road as well as raising the bridge or tunnel means a complete new road approach and alignment. This would not be a close the line over night or at weekends effort. This would be a major shutdown of the line for months on end. Even if there are parts of the network suitable for electrification there will be other parts where non electrified traction would be needed. Maybe third rail electrification might suit but that has other issues.
I do not think it is anyway "green" to ditch the 071 class purely on emissions based reasoning. They have been modified since first entering service and no doubt will continue to be. To mine raw materials, manufacture parts, transport them, take into account emissions caused by people travelling to work to make a "greener" loco it will be better to leave the 071 locos in service.
If we lived somewhere like India where they are building new railway lines, some specifically for freight, then buying new traction makes sense.