I can't see a route to any London destination being viable. Unless your actual origin or destination are very close to the airport, any time saving will be slight, and swamped by the much greater frequency of the hourly train. The only mainland routes which might work would be to SOU, Exeter or Newquay, where the train, although still frequent, is much slower, and involves a change of train (and train operator), and if travelling via London to Southampton, a change of station.