Why the Falklands 'Type 64' was not a Royal Navy class

Was there a secret Type 64 destroyer associated with the Falklands War?

No. 'Type 64' was a joke for the Type 42 and Type 22 pairing, not an official class or project.

The formal postwar sequence contains Type 61 frigates, proposed Type 62s, Type 42 destroyers, and other documented designs, but no official Type 64. The number instead came from adding Type 42 and Type 22 when the two ships operated as a complementary air-defense and antisubmarine pair. That combination could work well when its systems and crews coordinated properly. The Falklands War exposed the risk that the pairing had not been tested thoroughly enough before combat.