I put Greater Anglia’s app on my phone – quite a slick app as railway apps go – and ordered a GA smartcard which arrived a week later.
I then used the app to purchase an off-peak one day travelcard z1-6. I chose Harold Wood to Liverpool Street though any z6 NR to the appropriate London NR terminus will work (Brentwood or Amersham for z1-9). The default fare offered may not be the Travelcard but there’s a selection to choose one. I then used GA’s app and my NFC-enabled phone to load the ticket to the GA smartcard.
Yesterday I boarded a TfL bus near Feltham and with some trepidation and my Oyster card handy I touched the smartcard on the reader. It beeped and the light turned green! I then travelled around London on bus/tube/DLR/NR and the smartcard worked fine all day.
It was such a relief to be able to travel around without worrying about maximum journey times, when/where I needed to touch out, OSIs, or the nonsense at Wimbledon. The only disadvantage I can see is cost: Travelcards, especially anytime ones, cost more than the daily cap. For me, the convenience is worth the premium.
So there’s the solution for anyone like me who’s long had a requirement for a one day Travelcard on something other than paper. Hopefully this will help anyone else with a similar need. Thanks to Jonathan Maddox for asking the question and to Alan (28481k) for the solution.