Well, if to speak about rail options from London to Paris, then it's basically only one direct option - high-speed EuroStar train from St.Pancras station in London to Gare du Nord in Paris. And if you have relatively fixed date/time + can purchase your tickets some time in advance, then it's not very difficult to catch discounted tickets starting from only 69 GBP return (or 39 GBP one-way) - just visit official
EuroStar websiteFor travelers who want some adventure it is also possible to consider trip from London to Dover by train, then ferry to Calais and again train to London, but such trip will take much longer time and won't be cheaper, thus will make sense only if you have time & want something different then taking EuroStar train