In the past Australia bought two River class frigates that looked more liked Leanders than the previous four that looked liked Rothesays. They ordered four Adelaides from the USA than built two more several years later. Its not a bad thing to do, I think it helps stretch out block obsolescence, all of the ships coming to their end at the same time. Stretching out programs seems to give future governments more choice in their replacement. Since Australia bought eight new Anzacs, they had the choice to buy only three or four new air warfare destroyers to replace six Adelaide class FFGs. Notice the flip flop, from six GP frigates to eight FFHs, and from six FFGs to 3-4 DDGs. The new 8 GP frigates are much better than their old six GP frigates, while in the next decade their 3-4 DDGs will be much more powerful than their old FFGs.
So its not a bad idea to wait a few years to complete the class of warships. That is, if there is no hurry to do so with older ships being decommissioned faster than newer ships. While many think the UK has gone down from 12 destroyers to 8 destroyers, I look further back and see their old County class destroyers being replaced by Type 45s. The Type 42s in my opinion were more like FFGs.
Of course, with a large number of Type 23s replacing in my mind Type 42s and Type 21s, it will be interesting what the new class of frigates will look like, because in my warped sense they will be the ships replacing the Type 23s which replaced the Leanders. One thing is for certain, all of the British new amphibious ships are better than previous ones, and the upcoming CVFs will be a marked improvement to the Invincibles.
Frankly, I see good times ahead for the Royal Navy. I agree, its better to get the carriers bought before returning to destroyers, frigates, and submarines. I see the F-2000 designs the UK has exported more than enough to replace a large number of previous frigates. Well, I would like to see the home product just a bit larger than the export models. Maybe they should be called Type 24s or Type 25s. The simple truth, and the USN has confirmed this, in the future there is going to be a need for more littoral combat ships than ocean escorts.