I think the rationale for a LGW service is that if your destination is South London, once you have got to King's Cross, your adventure has just begun...
Is true. I live in South London and often make the journey to Leeds often by air, rail and driving. I used to fly from Gatwick when the service existed and ditto Docklands but I tried to avoid Heathrow which is a ghastly place to get to.
South London and the Gatwick hinterland must have a catchment way in excess of Glasgow, Edinburgh and Bristol and they seem to be successful enough.
Now Heathrow is pulled Gatwick and Docklands may well be viable now.