Originally Posted by
ajeleonard
Would make more sense to fly to Paris since you are already at the airport
But not cheaper. Current Eurostar cheapest price for May 26 at 14:22 is 44 GBP. Cheapest flight on that date after 10am from LHR is 74 GBP and only in the evening + OP still has to reach the city center from CDG or ORY which will add another cost (cheapest option around 9 GBP). Strangely enough, 2nd cheapest option is Iberia via MAD
So as long as the transport cost within London is less than 35-40 GBP, the train is the cheaper option.