Anyone needing cheap Advance train tickets for longish train rides around Christmas should start looking now. As I write (9th October) most train companies have released allocations for Monday-Friday up to 23rd December, a few have also released Saturday 24th and the following week to New Year. The gaps should be filled soon, keep an eye on the list at
http://www.nationalrail.co.uk/times_fares/booking_horizons.html

Beware that where Advance tickets haven't been released the sales systems won't tell you, they'll just offer some variant of Off Peak as the cheapest, the calendar for those is up to the end of the year as they don't need allocations just a timetable.