When I was crew there the flights would have significant amounts of those from north of the border onboard. The school summer holiday period lasts longer at NCL probably more than any other English airport due to those crossing the border seeking cheaper flights due to the school holiday periods being different. Similar to half term too. People can save hundreds by driving a couple of hours. I doubt APD will have an impact. First you need airlines to pass the saving on to the consumer and second it is not that much of saving by the time you've factored in fuel, motoring costs and hassle factor.