It doesn't matter how much better the Air Defence system is, or if the Radar is 5 times more powerful in terms of simultaneous contacts, 12 hulls can be in 12 places at once, unlike 6, which can only be in 6 places at once.
FFS, you can pay for an increase in development aid to countries with nukes and a very large economy (thinking India and China here) or to a crackpot pseudo-democracy with very limited freedoms (thinking Pakistan), but you ain't gonna actually pay for extra destroyers, even after Libya? Even just two more destroyers would be better. And another Astute boat would be nice as well, I've heard a fair few tales about the T-boats being overworked, and no increase in SSN numbers is planned.