The whole conflict is stupid, if the templars weren't incompetent, and wanted to actually finish off the elves, and wasn't a fantasy story, they would show up at their gate geared for war with some APCs (I'm sure you can get those in Neverwhere), take care of the guards, go in, find their city with a drone (shouldn't be hard), then bomb the shit out of it with artillery until nothing remained. If they wanted to get really nasty with it, they could use chemical or similar weapons too. Geneva convention doesn't apply there. There, problem solved. But I guess they will just fight with swords