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In the case of the UK, the earliest bookable tickets offered by National Rail are Advance Tickets. These are discounted early-bird single journey fares, available in limited numbers and with strict conditions. Advanced tickets are valid only for the selected journey (although they are changeable prior to departure) and non-refundable.
Turns out that National Rail has a webpage showing the dates in which Advance Ticket sales open, broken down per subsidiary. This page is automatically updated every day, since Advance Ticket sales open roughly as early as 12 weeks before departure.
Note that the train journey will appear in the Journey Planner, and hence be available for reservation, only after the train timetable has been finalised and published. Moreover the train timetable is dependent on engineering works. This is to say that the opening of the Advance Ticket sales does not guarantee that a ticket for the journey can be booked.