Print at Home tickets good for 2-for-1

Don't forget you can now collect National Rail tickets at Heathrow.

Print at home depends on the train operator and many of those limit it to advance tickets. Not local tickets costing £2.30.

I would also advise getting a ticket for a short journey to a London Terminal station rather than between two non terminal stations in London. You can more realistically pretend at any Central London attraction that this was your ticket to London although everyone will know what you are really doing.

Very few routes in the UK are available as print at home tickets. It is restricted to a few long distance (Intercity) routes where the ticket inspectors have been equipped with scanners. If you are doing a connection to a local train, hard luck.
Local trains like the one you quote, you can only buy tickets at the station, at the manned ticket office (if it exists) or from a machine.

I doubt anybody ever buys a Queenstown Road to Vauxhall return paper ticket. Like all trains in Greater London, Oyster Cards are valid on this route, which will be what "everybody" uses.

Thank you so much for the information. It had totally escaped my notice that you could collect National Rail tickets at Heathrow. I will do that instead of trying to find a print at home. I'll also take your advice to have the tickets end at a London terminus in order to be less obvious.