Surplus 701s will most likely be used on current 8-450 Alton & Basingstoke diagrams to allow those 450s to make sure the 458s aren’t required. SWR have said they, disgracefully, plan to mothball them.
I thought that class 458 units where being kept to work Waterloo - Portsmouth services in place of the class 442 units?
I wonder if it would make more sense to get new 120m trains for mainline services to the Kent Coast and backfill the Networkers with existing Electrostars.
Unless Alstom build a version of Aventra that is shorter than the 5 coach class 720 units, I don't think that they have an answer to the problem. The only real answer I see is Stadler Flirt, which in four car length is 80.7m. However, the units will have to have a gangeway at the front so that they could couple to other units and enable walkthrough when coupled.
The only other answer is CAF Civity where four car units are 94.75m.
Other possibilities are Siemens Desiro City at 94.70 for four carriage units or Hitachi AT200, but I think that these maybe a bit too long though, but would make sense since SE already operate the AT300 class 395 units.
With regards Siemens, unless there is capacity at there factories in Germany I don't see them building the new trains as they have 94 tube trains that are being built between their sites at Goole and Vienna.
Alstom at Derby have some capacity prior to building the HS2 trains, but I don't think that it would be enough time to be building 100 plus trains. CAF will have capacity after the 10 Civity tri-mode 10 car units have been built.
The only other option I see would be if the units where built be another manufacturer other than those mentioned above elsewhere in the world, like for instance maybe China since the UK Government seem happy to do trade deals with them.