A little calculation ... how low cost will BMI have to be to compensate for the taxes at Heathrow?
flyBMI.com : LHR-AMS-LHR: Taxes & charges: £50.40 return
easyjet.com: LGW-AMS-LGW: Taxes & charges: £10.00 return
(So 2 or more passengers travelling together, living 'next door to' Heathrow would actually be able to pay for a (cheap) minicab to Gatwick with the difference!)
(and before anybody suggests it, I have no financial or other interest in 'promoting' the boys&girls in orange)