Type 22 were technologically more advanced and more delayed. The Type 21 were built between 1969 and 1978, the first batch of the Type 22 between 1975 and 1982. The first Type 22 with Sea Wolf was commissioned a little more than one year after the last Type 21, the first Type 21 was five years earlier than the first Type 22 (1974 vs. 1979). It was very obvious from the beginning that the Type 21 will get with the Sea Cat an outdated missile and that a better missile will be available. The decision to built small cramped and easily overloaded ships, which could not be updated, was simply a very bad one and resulted in several really bad designs - and the Type 21 was one of the worst. The restrictions of the Type 21 were not by design, but restrictions caused by too weakly built hulls, i.e. restrictions to prevent cracks to form.
For sure, the Type 21 were the cheap type for patrolling, the Type 22 the sophisticated ASW frigate - similar to Type 31 and Typ 26. But in case of the Type 31 the RN did not make the mistake to buy an overpriced small cramped outdated ship (an upscalled corvette as the Type 21), but have chosen a proven spacious and cheap design.