Travel via the continent on an overpriced open-jaw ticket, with the inconvenience of flying indirect just to save £80 quid...? I just don't see why anyone would or could be asked with that.
Not everyone can fly directly anyway. If you live outside of the London area, chances are you have to fly to LHR or LGW to connect with your long haul flight.
Fly from the regions via CDG or AMS and save the tax. The tax system must be hurting British carriers.