I think the origin cannot be a London Terminal as that then causes an error when looking for default journeys, which are to/from London Terminals. Any other station should be fine, even some LU ones, such as Heathrow Undergd.
In all honesty, this is a turn up and go journey, I'd use oyster for this. We've had many discussions about Tickets on Departure availability at stations. A quick look at Google Maps, if you head over to South Bermondsey which isn't that far away, you can collect tickets there if you wish.
What ticket are you considering buying online for this journey? Is there any need to buy it online, could you buy it at the station on the day of travel?
I think you'd be better off using the RSH voucher for a longer journey, unless you are willing to travel from South Bermondsey or plan far enough in advance to get the tickets posted out / collect in advance. But London lives on the turn up and go culture. I wouldn't bother with the RSH vouchers especially if you live in the London area.