You are right about the need to maintain public confidence. The police DO know that. They use consultative groups, advisors and community contacts far more than they did, certainly in relation to "mainstream" crime. A "Community Impact Assessment" forms part of all operational planning.
I suspect that there is still some reluctance to use these measures in relation to terrorism matters - some of this can probably be justified but some of it is probably unfounded. I think they could probably do better. If there had been a thorough Community Impact Assessment and associated strategy, I would have expected comments such as those made by AC Hayman to have been made within hours, not days, probably fronted through a local senior officer working closely with community contacts for who a briefing was held after the event even if a pre-event briefing was not considered possible (as it may well not have been for obvious security reasons). Hopefully this incident will inform that learning.