But it was said to be due to a nasty storm
The two are not mutually exclusive. A storm would be perfect cover for a black ops mission.
However, the Russians are not fools, and should be well aware of the risks of such attempts, so should have corresponding security measures in place. The easiest would be not to store the missiles on deck where a pair of bolt cutters and some shoving might be all that is needed to send them overboard.
However, we must also consider the risks and costs of such a mission.
Getting commandos onto a moving ship in a storm unnoticed is going to be quite some challenge.
Same could be said for further breaching Russian security to get at the missiles.
That is a classic low probability of success mission.
Now weigh up the risks and costs of getting caught.
These are Russians, escorting top secret missiles, so odds are they are going to shoot first and ask question later if they spot intruders.
How well would a bunch of dead SAS commandos on a Russia ship in international waters play in the world? It would be far worse if said dead SAS commandos killed some of the Russian crew and/or security detail in the firefight.
That’s at best state sponcered piracy, and worst an open act of war.
On the other end of the equation is China, who the British are pinning a huge amount of hope on for their post-Brexit economic survival.
Needless to say, Beijing would not be impressed with such tempering of their newly purchased missiles.
Finally, we need to consider the prize.
These are export missile, and so would be different from what the Russians are using themselves.
What-more, the Russians have helpfully shipped a domestic S400 battery to Syria.
Why go to the risk and trouble to trying to steal export missiles when western intelligence could instead try and get the real McCoy Russian domestic signals by provoking a response from the sets in Sgrow instead?