All this getting our knickers in a twist over whether or not security would be compromised is just noise... the fact is, as mentioned above, that the money raised by a sale of NATS is peanuts compared to the money the government will get if they hold onto their share and continue to get dividends.
The sale price would be surpassed almost within the term of a government. The sale does not, apart from getting a quick buck, make any financial sense.