The option to purchase Indirect trains on the Eurostar website will be available around 90 days before departure. I paid £ 71 return for Adults and £67 for children, although ticket prices seem to be more expensive on some departure times. It's worth checking a few days before that 90 day mark as I managed to get mine 3 days earlier.
I booked mine in November for my trip next week, there are about 6 or 7 trains most days of the week and purchasing them for your dates should be no real issue as it's off peak. You're obviously looking at the opposite way round, but it makes no difference, you can still book on the Eurostar website 90 days in advance for both legs of the journey as one price. Test it with some dates that are available over the next 90 days for prices, it probably won't change too much between now and when you can book your dates