I would feel much happier with the little Guerreros onboard a British nuclear submarine than at almost any public event in Britain. Well, as long as they're kept away from the stokers.
Submarines are ships of war - designed and frequently used for battle. They contain things that go bang. Sensibly managed, however, visits from the general public should be encouraged. They paid for it after all and they find these things fascinating. The risk of someone going crackers with a firearm is present in any such environment, but given that I think it is unprecedented (I could be wrong), I'm pretty sure that it has never and should never feature in a risk assessment of such activities. Driving your kids to school is inherently more risky than letting them walk - yet many of us do that every day. You cannot legislate for a lone nut.
Psychological assessment for the suitability to handle firearms is achieved in the RN through the recruitment and training process and the Divisional System of supervision and leadership. Submariners are a highly professional bunch and that someone seems to have slipped through the net is unfortunate and deeply, deeply sad for the individuals and the RN as a whole, but it is not a reason to change a system that clearly works very well.