Eurostar is running another public sale from tomorrow (20th) through to December 2nd.
However, if you use this link, you have access to the sale before the general advertising starts tomorrow.
Here are the terms and conditions:
You will pay £54 return to Paris, Lille, Disneyland Paris, Brussels or indeed any station in Belgium, in Economy.
You must travel from 6 January to 12 February, excluding 31 January to 2 February.
2-4 people must travel together
There are 5,000 pairs of tickets available at these prices, and some peak services are not available. That said, if you book today you will have substantially greater choice compared to what is available when the advertising for the sale begins tomorrow.
At the time of writing (6pm Monday) I could find, for example, Paris outbound on Friday 24 January (all trains before 2.30pm) for £27 per person outbound. The latest £27 each-way ticket on Sunday 26 January was 12.31, but for an extra £17 each you could return until 16.13.
Remember that if you have an American Express Platinum card you can use Eurostar lounges just by flashing the card. However, you cannot bring in a guest, so your companion would need to be your supplementary Platinum cardholder.