When buying train tickets (advance fares) IPS-EDB out via PBO with a split, and return via London, I found that I could only have Ticket collection at a ticket machine. I suppose it's simpler and less confusing to have the whole order as the same delivery method than having part as ticket machine delivery and other parts e-ticket.
Separately, there are some cross London journeys where you are not using the tube. For example direct through Cambridge to Brighton or Woolwich to Cambridge, where you can use the Thameslink core. In either case it's not possible to use e-tickets - though I have found an example of CBG - Tottenham Hale - (tube) - Victoria- Brighton where the e-tickets appear to be available via the raileasy site. Maybe there should be a cross London but not tube fare, e.g. if you are cycling, have a through National Rail train, walking, or are paying for the tube on contactless etc. Oh wait that makes ticketing yet more complex. Probably simpler for TfL to update all their gates to support e-tickets.