I have a railpass which gives me a discounted ferry ride from Calais to Dover and back again. But, what's the cheapest way for me and another adult to get from CDG Airport in Paris to Calais, and also from Dover to London?
Why ferry?
The cheapest way is to skip the ferry altogether and ride the Eurostar all the way from Gare du Nord to St. Pancras (King's Cross) in London.
www.eurostar.com for information
One way tickets with advanced purchase can be as cheap as 50 euros this time of year.
Assuming your railpass is good in France, it will be good for a discount fare on the Eurostar. See the fares list at www.eurostar.com.