NCL is hardly unique in that regard.
If a UK traveller books a package holiday abroad then the chances are he'll be booking with a UK company, which supports jobs in the UK and contributes to the revenue. It will support jobs at the airport, indirectly and directly. Debit, credit cards, mobile phones used abroad will generate profit streams back in the UK too. The UK hotel and restaurant trade misses out, but I bet the analysis is interesting when its looked at in the round