TBH, even the abolition of APD would make little difference to the price of flights. If you take a typical price breakdown (VA LGW-MCO, out 17/08/14, return 31/08/14) at £1,436.97:
Fare 1: Carrier VS RTWGB LON to ORL (rules)
Passenger type ADT, round trip fare, booking code R
Covers LGW-MCO (Economy) £414.50
Fare 2: Carrier VS Y2UK ORL to LON (rules)
Passenger type ADT, round trip fare, booking code Y
Covers MCO-LGW (Economy) £692.50
VS YQ surcharge (YQ) £208.00
VS YQ surcharge (YQ) £5.00
USDA APHIS Fee (XA) £3.00
US Immigration Fee (XY) £4.20
US Customs Fee (YC) £3.30
United Kingdom Air Passengers Duty (GB) £69.00
United Kingdom Passenger Service Charge (UB) £12.27
US International Arrival Tax (US) £10.50
US International Departure Tax (US) £10.50
US September 11th Security Fee (AY) £1.50
US Passenger Facility Charge (XF) £2.70
Subtotal per passenger £1,436.97
It's only £69 out of the total price, so any reduction wouldn't really make much difference whether the fare was £1,100 (as here) or £500.