We will be traveling in September to the UK. We will take the train from Oxford in to London and then a week later, leaving London to Edinburgh. Can we take advantage of the 2 for 1 offers while in London that I see advertise on the National Rail website?
Yes, on the day from Oxford to London, with the segments you have outlined. While you are in London if you use one day Travelcards which are issued on paper from National Rail stations you can during the time the Travelcard is valid use the 2 for 1 scheme. Travelcards issued by Underground stations are not valid.
Your paper rail tickets to and from London qualify for the Days Out 2 for 1 discounts. Be sure to hang onto your inbound ticket, do not feed it through a turnstile, go through a manned gate. You have to present both the inbound and the outbound ticket along with a Days Out voucher to receive the discount. The first few paragraphs document the ticket rules: http://www.daysoutguide.co.uk/faq
Chip is absolutely correct. I hadn't realized that people could have a single ticket to London and a single ticket back and still use the 2-4-1 while they are in London without the requirement to get Travelcards - But they CAN. That's great news...