With Heathrow Airport departing charges at £41 per passenger plus APD at £13, you need to be selling for at least £54 one way ex LHR or else you are increasing your losses by putting extra passengers on that aircraft. I'm not sure what you would see as a "knock down" rate, but Heathrow is virtually the last airport on the planet where that type of commercial behaviour is sensible or practical.
Ironically, three months after Little Red goes, Heathrow is reducing passenger charges for domestic flights. What a stroke of genius... Kill off bmi, help make Little Red unviable and then reduce the fees back closer to where they were, just as one operator restores a monopoly on all domestics to LHR. Now who thought up that one...?