There is good reason to believe that reducing APD or even removing it would entice airlines to operate regional routes. I understand that Dublin got the nod over Manchester for some transatlantic traffic for example because of APD.
Heathrow will always be the focus of air traffic for the U.K. and I have no problem with that however there should be a middle ground that encourages more people to fly from the regions and at the same time allowing continued growth of LHR as 'the' hub.
However I don't think doubling APD from London is the answer, reducing it elsewhere is, however that's not what the government want as it looses them valuable revenue and there is the real problem it's too valuable a revenue stream for the government. They do not have the wit or intellect to realise that this tax is actually damaging not just a valuable industry in the airline business but tourism as well. Taken overall one wonders whether the revenue from APD brings more than that which is lost in both the tourism and airline industries. I suspect we could all make a reasoned guess on that.