pr00ne
Perhaps they are paying off the T23's rather than more T42's so as to retain a balanced force, you know, capable of more than just one task.
That was my point. The T23 is more flexible (equipped with sensor and weapons for dealing with surface, submarine and air threats, wheras the T42 is basically a platform for Sea Dart and can only really function in an anti air role. It has no anti submarine weapons (other than the Lynx it carries - can't carry two of them or a Merlin), and no dedicated anti ship missiles.
As for air defence of the UK during the Cold War, I agree. But that was then and this is now.