The Independent is reporting that Eurostar is offering its lowest-ever fare: £19 one-way from London to Lille or Paris or Brussels. The tickets are not easy to buy, however:

• The offer is available only for Facebook users, from snap.eurostar.com.
• You must book at least a week in advance
• You cannot choose the departure time for the train — only opting for morning or afternoon travel. Two days before travel, Eurostar will confirm the exact time of the train
• Bookings will open on Monday, 21 November and close eight days later.

Also you may have to sit on a tip-up seat for the entire journey depending on availability. For 19 pounds, I would be prepared to stand!