But to be fair to them, maybe it does partly solve a problem. If you can take LON destination domestic traffic out of LHR and route them through NHT there could be the possibility of slots being opened up for longhaul traffic. Connections to central London are there through Ruislip and Hillingdon so although self-serving, it could be an interim move.
Nah it just makes the competitve route at LCY much busier. The Piccadilly Line from LHR is bad enough without having a go on the Central Line to Northolt. No one flies destinations split by p2p and connecting on different flights, they go hand in hand. You force move anything to NHT from LHR you kill connectivity which is the selling point of LHR over LGW, STN and the rest.
You would quite spectacularly put any such carrier at a copmpetitive disadvantage and face immediate legal proceedings.