Great Northern's Super Off Peak tickets are good value. How many of you are travelling? If there's 3 or more than you can get the ticket as a Groupsave and save 34% as long as you all travel together.
Depending on what you're doing when you get to London you might want to buy the Super Off Peak Day Travelcard for £18.60 which gives unlimited travel on trains, tubs and buses within Zones 1-6. If you're doing more than a couple of Underground journeys it'll probably work out cheaper plus there's no need to top-up.