After a successful LCC campaign, Eurostar allows bikes to be carried on its trains.
(1) If you book in advance, and pay a £20 additional fee (each way), you can take your bike on the train with you. The bike service is only available between London, Paris and Brussels.
(2) You can also take your bicycle for free on any Eurostar train if you can fold or dismantle it into a bike bag or neat package which isn't bigger than 120cm x 90cm in its larger two dimensions
If this is the case, you carry your bike on as luggage, and stow it in the appropriate area in the carriage where you're seated.
(3) Bikes can also be carried as registered baggage on any Eurostar services. In this instance, you present your baggage at the Eurostar terminal, but aren't guaranteed that it'll travel on the same train. Eurostar guarantees it'll arrive at your destination within 24 hours.
Contact Eurostar on 08705 850850 or via the link on the right.