Well, it turns out that it is cheaper to fly to Paris from London (tickets going from $61 to $88 - yes, dollars - on Kayak searches), than to take the famous train ride with Eurostar (cheapest ticket was 77 EUROS!!!! ) If you get a sale, you can get to Paris for 48 EUROS, but it has to be on the days they have a sale. We don't happen to be traveling on those days.
Of course the flight is shorter than the train ride as well. On the other hand, it might take less time to get to the train station than to the airport.