I know the Eurostar is the best way from London to Paris, but does it even go the other way (from Paris to London)? If not, how can you get from Paris to London fairly cheap?
If you cannot get one of the cheap Eurostar tickets (50 euros one way?), then the next best option is probably EasyJet via Luton (around 80 euros for the flight plus another 20-30 euros getting to and from the airports).
Eurolines bus. It goes from Galieni to Victoria. I think the price is about 35 quid, but the journey will take forever and be pretty uncomortable. My advice: With the change from 50 buy a strong bottle of something and try and sleep through it