firstly, its not at all the same, secondly you've conveniently ignored that British had said operations may extend beyond the "exclusion zone". it was a threat, it was destroyed.
this "sailing away" is horsecrap. it was an enemy warship operating around a theatre of war. one minute its bearing is away, the next it could easily be back towards. you know it was leaving the zone and not a tactical manoeuvre? no. you have a clear shot, if you're the captain of the sub...