To Paris from London

Q: I would like to know how to travel from London to Paris what is the cheapest companies (bus, railway).

A: going to Paris by coach and booking it in advance is probably the cheapest - see this link.

http://www.nationalexpress.com/p.cfm?n=gel&agentid=elhm

Eurostar is quicker but also usually more expensive (train from Waterloo).