I had a completely bewildering exchange with either a bot or a clueless (and probably offshored) Eurostar employee a few weeks ago about this. I was trying to find a ticket from Cambridge through to Avignon less than 90 days out, and I knew that they were available through National Rail.
(Although I know suspect engineering work between Cambridge and a King’s Cross - but not the perfectly viable alternative of Liverpool Street - was making it impossible to Eurostar to find a route)
Luckily Eurostar are kind enough to record and send you a transcript of these chats!
( w4m 35s ) James: Also, why isn't it possible to buy a through ticket from a non-London station? Cambridge isn't recognised as a departure point.
( 6m 18s ) Alexandru: As you can see here:
https://booking.eurostar.com/uk-en/...bound-date=2019-11-20&inbound-date=2019-11-23 Cambridge can be booked as a departure station to Avignon. You just need to type the first the letter of the station and select it from the drop down menu.
( 8m 57s ) James: It is not appearing. Can I send you a screenshot?
( 9m 14s ) Alexandru: Please access the link provided.
( 9m 44s ) James: Error message "Sorry, there are no trains going out on your chosen date. Can you go out another day?"
( 11m 1s ) James: Date is 31/12/2019
( 11m 8s ) Alexandru: Could you try to open the link in a hidden/incognito browser, as on my end the link works and I can see both legs of this journey.
( 11m 25s ) James: I've checked with NRE and trains are shown as running between London and Cambridge on that day.
( 11m 53s ) James: Likewise, 29/12, 30/12 - error message "Sorry, there are no trains going out on your chosen date. Can you go out another day?"
( 13m 21s ) Alexandru: The national tickets have a booking horizon of maximum 80 days. This means that you can book them with maximum 80 days before your departure or return date, if it's a return booking. So,this means that your national tickets will be available starting with 12th October for a departure on the 31st December.
( 13m 53s ) James: It's 20 October today.
( 16m 1s ) James: ?
( 17m 47s ) Alexandru: As I said, the booking horizon of 80 days is maximum for the national rail tickets. As you can see on the website, the tickets are not available yet for booking, because those tickets are not released yet.
( 18m 13s ) James: We have passed the 80 day booking horizon.
( 18m 17s ) James: 12 October was last week.
( 18m 24s ) James: We are less than 80 days from 31 December 2019.
( 21m 39s ) Alexandru: The booking horizon for the national tickets is maximum 80 days. The tickets from Cambridge to London are not released yet. Please check with the rail operator that runs trains on that route about date when the tickets will be released.