I don't think that the broad security arrangements in any HM Ships are a secret. At the end of they day, I'm pretty sure that the general public would be quite keen for nuclear vessels to be guarded by armed sentries. This is the case at all times with one or two very exceptional exceptions.
All RN Warships, nuclear or otherwise, have armed sentries whether in base port or elsewhere. I've counted the bloody bullets often enough to know.... Anyone who has visited an RN Warship will know this - it is not secret. They usually carry batons as well (not of the the Majorette variety, the "ouch my leg doesn't work anymore" variety). Obviously the level of defence is always under review and is closely related to the overall security posture at the time, but I can't recall any occasions during my service when armed sentries were not a minimum requirement.
What does surprise me is that a former Minister of the Crown is seeking assurances that there was no risk to the public (presumably because he read that nuclear=bad somewhere in the Socialist Worker). My advice would be to give Bob Ainsworth a ring if he's really that ignorant.