It is a complete non-story. The UK has already revealed back in 1998 the number of Trident D5 that is pools from the US Trident inventory. This was revealed in the Strategic Defence Review. The number was 58. All the US is doing is supplying the serial numbers for these UK Tridents. This will enable the Russian inspectors inspecting the US Tridents on US soil to verify the exact numbers in the inventory.
The UK during 2010 unilaterally revealed along with France and the US the numbers of nuclear warheads. UK numbers are 160 nuclear warheads operational with a maximum not exceeding 225. All revealed in Parliament. The UK has also revealed the number of missiles and warheads that they would deploy on the subs as part of the deterrent. All very much public knowledge.
Washington lobbied London in 2009 for permission to supply Moscow with detailed data about the performance of UK missiles. The UK refused, but the US agreed to hand over the serial numbers of Trident missiles it transfers to Britain.
...This appears to be significant because while the UK has announced how many missiles it possesses, there has been no way for the Russians to verify this. Over time, the unique identifiers will provide them with another data point to gauge the size of the British arsenal.”
While the US and Russia have long permitted inspections of each other’s nuclear weapons, Britain has sought to maintain some secrecy to compensate for the relatively small size of its arsenal.
Probably not a good idea to have the standard knee jerk reaction to such stories without knowing the facts.