Most rail companies withdraw all advance purchase offers from roughly one week before Christmas until just after New Year. So it might not be possible to get a cheap ticket for your outbound journey.
However, it's possible that CrossCountry might decide to accept advance bookings for the week prior to Christmas. At present they're not accepting advance bookings beyond 8 December. I suggest keeping an eye on their website to see when booking opens for dates beyond this:
http://www.crosscountrytrains.co.uk/
(If you're lucky you might get a single ticket for �12. That's what CrossCountry's lowest advance purchase fare is for Saturday, 8 December:
http://www.nationalrail.co.uk/ )
Chris