I realise that militaty Internet forum debates can often become a game of Top Trumps, wishing for bigger and bigger caliber weapons. But in the case of protecting UK naval bases from terrorist attack, any weapon system is the last resort and must be able to be used in locations often crowded with merchant ships, ferries and civilian pleasure craft.
Mostly ignored/forgotten because they are much less visible, but far more useful, are the intelligence, surveillance, command and control capabilities. If the only thing deterring a terrorist attack was the weapon system, we would be in deep trouble - even if it was a Mark VI patrol boat with 25mm chain guns.
The best and first defence against terrorists is for those who know the terrorist (friends, family) to feel safe enough to be able to report odd behaviour to the police and to trust that any actions taken will be both safe and fair.
The best and first defence against terrorists is, therefore, the UK culture of fairness and justice for all.