NI number
Any UK employer must be sure that you have the right to work in the UK, otherwise the fines are massive, a NI number is a way to do that, but of course if you have the legal right to live in the EU then you have the legal right to be employed in any contracting state within the EU.
So just put you social number down from your own country and remember this is more likely to stop your job prospects than your flying skills or lack of.
Working really well this EU project????