You could take a Euroline coach. Check eurolinesdotcom for details. It might be somewhat cheaper than Eurostar. It takes longer, but you would have a ferry trip as well which is fun.
Yes, you can take ferry to London from Paris.. there are many places that you will visit while travelling to Paris like London to Calais, Boulogne, Amiens and then comes Paris. There are many favourite destinations Paris and France. we can visit and explore the travel trip..