If there is a home for any 769s, I'd have expected the effectively unused GWR ones to have found it ahead of the ex TfW examples. The GWR ones also have OHLE and 3rd rail capability, widening their sphere of usefulness.
All you can possibly do as I believe will be done with the GWR class 769 units, is place them into storage and hope that they don't rust away before they are needed.
But I suspect that given the age of the donor class 319 units, that they will be scraped.
For instance the GWR ones, would be good to replace the class 171 units on both the Uckfield and Eastbourne - Ashford services. But with 9 ft 2.9 in width maybe too wide for some of the tunnels, especially on the Uckfield route. But would you want to replace units that are 20 years old with one that are 36 years old? I think the answer there would be no, especially if the parts get more difficult to obtain.
Also, as JonathenH has pointed out above, you have the reliability of the units. If i was at GTR and new stock was able to be purchased, then I will more than likely look at new stock from Alstom, Hitachi, CAF or Stadler where more than one power can be used and if diesel power is used initially with said units, that it can be replaced to be used with battery power at a later date. But there is no funds available for that to happen, hence GTR have shortened the class 171 fleet to just what is required for the limited services that are run both Uckfield and Ashford routes.