Same-Day Return Ticket London to Cambridge and Back?

An off-peak return from Kings X without any railcard is £23. This is valid on any train except before 9:30am Mon-Fri. At this rate it doesn't pay you to buy any railcard if this is your only journey. £33.10 is the any time fare for travel before 9:30. FCC don't offer advance fares. They do exist from Liverpool Street starting at £6 one way, but the journey is around half an hour longer than from Kings Cross.

Yes, it's the Greater Anglia service from Liverpool Street. These trains can take as little as 1 hour 10 minutes to Cambridge, but the FCC services are faster, as Marco points out.

OK, I may have answered my own question. BritRail website was not very helpful, but First Capital Connect came up with a same-day return fare of €66 for the two tickets. That is for a set train outbound and open return the same day. Ran the price with a two Senrior Railcards and the same-day return offer disappears; the website only offered tickets for both outbound and return and the total of the two adds up to more than the same-day return without using a Senior Railcards ( which we would have to buy for £28 each). Am I missing something? Is the €66 for two the best price we can get? I checked different dates and it doesn't seem to vary with advance purchase. Can we get this same price the dayof travel, buying tickets at the station?

Lola, the cheapest 'Advance' tickets are only available as singles. For a random date in the second half of August I can see a £6 Advance single fare on the 09.58 to Cambridge and £6 Advance single on the 16.40 coming back to London. As you probably know, Advance tickets have no flexibility and can only be used on the train they are booked for.

Thanks, Kevin! Are those fares from the Liverpool St. station or from Kings Cross?