I don't think CWL - LCY is a real proposition due to the poor connections between the city and airport. However, my understanding is that the route is designed not to serve Cardiff but Swansea. Now that's a whole different game. The rail links west of Cardiff are patchy and hugely unreliable. A London trip from anywhere west of Swansea is an all day affair. For example I once went Camarthen - London on the train and it took 8 hours !
A service aimed at Swansea and West Wales operated direct from Swansea has a real possibility of success provided the new terminal at Swansea is a really "walk through" affair, keep V. Short check in times and keep the Car Park free ! Then as long as you don't go tech too often an ATR to LCY should get at least 50% load factor. Just have to work out if that's enough to break even.
Good luck to AW I think you've finally found yourselves something that might work.