The
Duchess Cardinia class of destroyers would be approaching the end of their service-lives at the start of the 2020's - facing a decision on replacement, two options were chosen for further refinement in the 2000's - a high-risk trimaran platform, and low-risk legacy, building on the
Challenger cruisers line - literally, in this case. One of the 2015 cruisers was delayed, and a prototype, the
Antimache ordered in her stead. (A sub-scale trimaran, like
Triton is currently serving as a training ship for the Imperial & Royal Naval College, the legacy of the alternative path not taken.)
The close of the CLG line will lead directly into production of the new frigate line, being based on (very) similar platforms, hopefully with little disruption and cost overrun. These ships are deemed 'frigates' due to a combination of their large size, as cruisers, and built to those standards, and an attempt to avoid political questions about why 'destroyers' and 'cruisers' are almost completely the same at this point.