Nightstopping isn't really expensive. However, it doesn't happen willy Nilly and I think we all now realise that domestic LHR isn't, and really shouldn't be, aimed at the point to point market, especially day returns.
Domestically, the service cannot compete point to point with the train. The route is aimed purley at the connecting market similar to that that exist on the MAN-LHR shuttles whose breakdown of passengers consist of about 80-90% transferring.