Type 21 was not a tidy replacement programme

Did Type 21 replace Type 81 while Type 22 replaced Type 12 and Type 14 frigates?

No. Type 21 maintained numbers through an available yard and design, while Type 22 covered more advanced work and Type 23 was intended for Type 12 replacement.

Fleet programmes are often driven by available designs, yards, and urgent numbers rather than a clean one-for-one succession. Type 21 was a practical way to sustain frigate numbers when an independent design and a free yard were available. Type 22 belonged to the more advanced end of the escort programme rather than acting as a tidy replacement for one named predecessor. The Type 12 family was instead meant to pass its role to Type 23. Shipbuilding capacity and immediate fleet arithmetic can disrupt the simple class family trees often imposed in retrospect.