The most important factor is that jobs will be produced that will benefit the unemployed from Harpenden to Henlow, St Albans to Silsoe, Luton to Loughton, Stevenage to Sandy and all the other towns, villages and hamlets local to the airport.
You forgot to mention Latvia, Poland etc. I trip on the staff bus reveals most of the jobs don't go to the locals but to Eastern Europeans who will work for less.