I did it two weeks ago!
Also the eurostar lands you in St Pancras so its a nice short ride to Euston for the onward journey. its e29 each way per bike and your other bags must be 2 and less than a certain dimension. The bike is sortof shipped separately on the same train.
There's a couple of different options for going London Paris depending on how you get across the water.
1, London, Newhaven, Dieppe, Paris. The difference here is that you basically go due south, get on a boat and go south again for 4 hours. So its fairly direct in terms of how the crow flies. Also a nice ride. Donald Hirsch's route is particularly good on its entry to paris and approach to the eiffel tower.
2, London, Folkstone/Cheriton, Calais-eurotunnel terminal (sangatte), Paris. http://www.eurotunnel.com/uk/traveller- ... ycles-one/
3, London, Dover, Calais, Paris. Again no idea but routes have been worked out.
The london routes generally exit central london via greenwich. if your heading for dover you could skirt north and head for a thames ferry crossing. If your going into central london then the canals and riverside trails may offer helpful routes.