You can buy a ticket at most one month in advance. It is generally not recommended to buy a ticket far in advance, since they cannot be refunded.

If you don't have a season ticket and can travel during off-peak hours, then you could buy an Off-Peak Early Booking One-Way ticket (in the month before your travel date). You need to buy this ticket at least four days in advance and this ticket seems to be the cheapest option. If you have to travel during peak hours, then a regular eticket is your only option. I checked Trainline, but they don't seem to have cheaper tickets.