Being in the London/South-East area you won't be able to split tickets to reduce the cost as much as, say, a CrossCountry ticket, due to the every route being a commuter route as opposed to InterCity. Similarly, TfL don't allow off-peak returns entirely within the London Zones anymore. For these two reasons, you won't be able to to much better than the basic fare (£25.50 off peak). If it was me, I would go by bus to Romford (is it still the 351 and then 498?), and then a return from there would be the far more manageable £15.70, but that is quite time-consuming (and awkward!).
I hope this helps; if it doesn't, I hope someone else can suggest something more useful
EDIT: Actually, following the above idea, an Off-Peak Travelcard from Romford is £7.50; all you would then need is a return from Feltham to Bracknell (£7.40). This would save 80p and also allow much greater freedom in the Travelcard area. Otherwise, you can look forward to the Jubilee Line from Stratford! Further, I think Travelcards are valid on the 498, so you would only need a single into Zone 6. But don't buy a Travelcard from Brentwood or Chelmsford; the cost is ridiculous!
