Reference your comment about some journeys being considered as unlikely to be made, and so not having fares defined. While there may be such unlikely journeys, saying they won’t be made is asking for trouble and dispute when they are. Perhaps if they can’t come up anything better, they should be set to £0.00 (which might concentrate minds).
In your particular example (of Reading to Piccadilly stations between Osterley and Heathrow), I don’t see these as any less likely than other journeys to outer London tube stations. And the London Tube & Rail Map doesn’t indicate SWR as likely route (and probably won’t until Oval reaches the line via Wokingham). So such Reading to Picc. jouneys may be made be made Hayes & H-Heathrow or Ealing B.-Acton T. (or even Zone 1) – which ever way, passengers will just expect to be charged an appropriate fare. As if they were travlling from West Drayton…