Seems like far too much investment for the minimal revenue the line generates, the situation with the 315 has only come about I suspect since nobody is willing to pay for an entire new unit just to run that branch. I think it's quite foolish personally, it'll work for a while when they still have spare parts for the unit but they'll need either two of them, or a 710 to be able to substitute for it when it fails, so 710s will still need to be cleared for the route. Otherwise, as suggested a 360/2 could be used if you lop out one of the trailer vehicles (don't know how practical that is to achieve) or otherwise perhaps a 360/1 or two could be retained with minimal conversion, but it'd have to be more than one unit or you're in the same situation as using a 315. The comparable upside is that 315s are entirely reasonable to scrap and there will soon no longer be any requirement for drivers to retain knowledge of them. 360s are units that will still exist on the mainline elsewhere for decades to come and have worked the area (if not the route) before.
In all honesty, even though it's wasteful, I think having an extra 710 for the branch in commonality with the rest of the fleet is the most efficient way of dealing with it. If you think TfL are poor now though, the imminent effects of brexit plus the NtFL order are not going to make that situation any better. I'm really not sure how the situation is likely to end.
