Due to the workload of the Admiralty design department in the 1960s, a private design (Type 21) was purchased as an interim stop-gap whilst the Type 22 was under development.
På grund af arbejdspresset på Admiralitetets designdepartement indkøbtes et privat design (Type 21) tænkt som et midlertidigt hulstopning mens Broadsword-designet var under udvikling.