Normally the 'duty-free' stuff for sale on planes is a rip-off, but I've just come back from Egypt on Jet2 and found that the prices of 1 litre spirits on the plane was cheaper than at the airport at either end! Current prices are (applies to their flights out of the EU ONLY):
Bacardi, Bells and Smirnoff (blue label) = £11 a bottle or 2 for £16!
JD and Bombay Sapphire, £16 a bottle or 2 for £20!
Combine these offers with the £5 off £30 voucher I just posted and you can get a damn good deal! 4 litres of Bacardi, Bells or Smirnoff blue for £27 - £6.75 for a litre bottle!