The U.S. and UK share the EXACT SAME MISSILE. The RN and USN Tridents come off the same line. Part of START (and SALT) was the number of launch platforms. BOTH sides are allowed to see the production records for the launch vehicles of the other country. The U.S. HAD to provide the serial numbers of the Tridents, the Poseidon's and the Polaris missile bodies as part of the treaties. This is something that dates back, oh, 40 years or so. The U.S. is not privy to the number of launchers or warheads per launcher deployed at any given moment by the RN. RN warheads are not built on the same line as U.S. warheads, hence the U.S. (and USSR/Russia) does not automatically have full data on the warhead performance or production totals
The U.S. has, under a series of treaties dating back to 1972, agreed with the Soviet Union/Russia to limit the number of launchers for nuclear weapons.
SALT & START have, as a verification element, a requirement that the parties have access to missile body production records.
This includes ALL production.
U.S. Navy and Royal Navy weapons are identical and are produced on the same line.
When a missile is decertified by either the USN or RN a replacement is drawn from common stores, with the decertified launcher returned for repair and then replaced in the common storage pool.
In other words this is a non story.