The EU are not going to let NI have no tax and the rest of UK have it. It casts as discrimination and UK gov will have a court case on there hands with EU. Other countries in EU are joined and some have tax and others don't.
Quote from the HM Revenue & Customs website...
APD is not payable on flights departing
from airports in the Scottish Highlands and Islands.
The following airports, capable of taking dutiable aircraft, are situated in the Highlands and Islands region for the purpose of this exemption:
- Barra
- Benbecula
- Campbeltown
- Inverness
- Islay
- Kirkwall
- Stornoway
- Sumburgh
- Tiree and
- Wick
Flights from other areas of the UK to airports in this region are liable to APD at the appropriate rate.
So APD tax differences already exist (albeit on a smaller scale) within the UK.