You should have no difficulty being offered to collect a ticket from a ticket machine (“Ticket on Departure”); there’s no need to wait for post.
Routes where it is possible to travel via London (even if you don’t have to) are not currently enabled for barcode tickets (of any form) because TfL won’t accept barcode tickets, for quite reasonable reasons. Solutions are being examined.
Southeastern don’t accept barcode tickets at the moment. Who knows, perhaps that will change?
Barcode tickets in the rail industry use the Aztec barcode standard, rather than QR codes.
None of my local stations have TVMs and despite growing passenger numbers and increased service frequency, there are actually now plans to install them, probably because the majority of pax use Etickets now.
Without finger pointing at individual TOCs too much, how can a national rail system have been left to get in a position where it's impossible to buy a ticket between two stations in England either online, at the station or even onboard the train?
Majority of times the train manager on GWR service from Cornwall tells me to wait until Paddington to sort my ticket out, a couple of others have insisted I buy a ticket to London and then get it refunded at the ticket office at Paddington and re purchase the correct ticket. There's also a lot of unmanned stations in East Anglia this is applicable too.
During disruption when I've had Etickets to Brighton/Worthing via Reading and Gatwick or via Westbury/Salisbury and have been diverted via Paddington/Victoria via underground, TFL staff have never given me any hassle with them.
Only time in the Southern region I've had an issue with them was at Angmering a couple of weeks ago when the Gateline assistant tried telling me it wasn't valid and apparently it was only a booking confirmation blah blah, and we don't accept them as somebody could print 10 copies off ect. The ticket was even issued by Southern, with their logo on it FFS. He eventually opened the gate but made his feelings clear, and even had the cheek to say he could have penalty fared me. TBH, I wish he had, because he'd have been the one that looked incredibly stupid.
There is a potential scenario where an Eticket could potentially be legitimately bought to a Southeastern managed station I think. Any ticket into Waterloo on from an SWR station to London Terminals is still valid to continue via Waterloo East to either Charing X or London Bridge. Correct me if I'm wrong.