Oxted Tunnel is very tight fit and mk3 based EMUs don't fit so 769s are a non starter. Hence battery electrostar (and some more 3rd rail) and also substation upgrades will be required between South Croydon and Oxted as the current EMU running maxes them out any bet whether Abellio/Porterbrook/Eversholt checked with NR on what the line could take before possibly suggesting 171 transfers??? (and are willing to fund any upgrades needed)No, but clause 5.9.38 b) ii. suggests they are a compliant solution because there is time to find a solution (2 and a half years before September 2021) rather than because an obvious solution already exists. That said, "solutions" such as Class 769s, battery 377s and similar are out there and could probably be implemented in time for them to come off lease.
With regards the 360s - AIUI the 379s have higher leasing costs, which was a factor in Abellio choosing to replace the 379s with the FLIRTs in the first place, so one would assume Abellio considered the 360s a better fit in terms of fleet size and leasing costs.