British firings primarily test the submarine launch system, while the United States performs certification work on newer Trident missiles. Britain economizes by firing missiles near the end of their service lives. That choice increases the chance of a missile malfunction even when the submarine's launch equipment performs as intended. A failed test therefore does not necessarily show that the entire deterrent chain is broken.
Notable Point · 2:26:45-2:27:44
British Trident firings test the submarine with old missiles
The stream explains why unsuccessful British Trident firings do not necessarily show that the deployed missile system is unreliable.