I posted this in the General Europe forum,but though it should go here too:
Loco2.com (my preferred booking engine for train travel in Europe and the UK) has created this delightfully presented and very informative 'infographic' about how far ahead to book and which are the cheapest days to travel.
In the UK you can buy tickets 3 months in advance usually at a reduced rate than paying on the day of travel
This page on the National Rail site lists the dates for which tickets are available. Sometimes tickets in the week can be booked longer ahead than the weekend, if there are timetable revisions still to be finalised for engineering work.
