It isn't specifically mentioned on their website, but Eurostar have announced in press releases and promotional emails to regular customers that they have expanded their booking period to 358 days (or almost a year) before date of travel.
We don't know yet if the cheapest fares will be made available at the beginning of the booking period, but this may mean that it will become more challenging to obtain tickets around national holidays, school vacations and other high-demand periods, as well as making it more of a gamble to buy the cheapest advance tickets if you intend to continue your trip on another train.