Air France is currently running a promo on flights between London and Paris, with returns available at an impressive £78 all-in.
You need to book before 1 March 2015 to benefit from the £78 special fare. The £78 fare is currently widely available. By way of example:
Don’t forget that Eurostar continues to offer a sizeable number of returns from London to Paris at £69: fares which are very easy to search for. If faced with the plane or train in these circumstances, I’d choose the train every time – but I appreciate views may differ here.