To be honest when you see todays problem it doesn't make any sense. I can see why the mk3 stock doesn't fit as it is wider. But the Electrostar's do so to solve todays problem electrifying the rest of the line and using the electrostar's does seem to solve todays problems. If they don't have enough then we have enough 3rd rail stock about to go to the breakers, or they can build more for lines where the loading gauge isn't as restrictive to free up the Electrostars for this line.
The future I guess the can specify the rolling stock must fit this line, they have done it before. But if that makes a non-standard expensive build they may need to look a that again. But I guess that problem is at least 20 years away on this line.
The problem is at least 20 years away and within that time other solutions will present themselves such as trains like the Coradia iLint currently on test in Europe.
If you read any Network rail documentation on decarbonating the network, many say that there is finite capacity on many lines. By making Oxted Tunnel single track, you are making that capacity even finer.
There is no need to be paying extra cost, if as Hwl has pointed out there is existing newer stock based on the 20m Aventra that should fit. But if you did single track the existing Oxted tunnel, as Zwk500 has stated you would need to be creating a single track bore, so that the current capacity could be maintained. Depending on the costings, but you may find that changing the Infrastructure costs you more than buying a fleet of trains off the shelve, that can cope with using Oxted Tunnel in it's current state.