Well it's not exactly a secret, but you'll never see an advertisement for Eurostar Snap, the special site managed by Eurostar that sells heavily-discounted, last-minute train tickets between London and Paris (and Lille and Brussels). There are a few constraints:
- you have to purchase tickets between 7 and 21 days before you travel
- you can only choose your dates and "morning" or "afternoon", and you'll be informed two days before travel the exact time.
- these tickets can't be refunded, modified or transferred to another person
On the plus side the tickets are usually just £25/€30 one-way between Paris and London. Some holiday travel tickets are as high as £85, but that's still lower than the prices listed on the official Eurostar site. So now there's no excuse for a quick skip across the pond for Afternoon Tea!