One "something" that should not be done is to waste more effort on the Communications (Surveillance) bill, as was called for by a retired Home Secretary within hours of the atrocity. Its irrelevance to such cases is demonstrated by what is emerging about the build up to the very public murder of a squaddy within sight of his barracks by perpetrators who were not only known to MI5 but seen as potential recruits.
I doubt, however, that "Keep calm and carry on" is a serious option, unless it is accompanied by effective action to deter, not just detect, those who might wish to obtain equivalent media cover for similar acts (whether their motives are racial, religious, animal rights or ...).
We will soon see demands for effective action to deny terrorists the oxygen of publicity and to drown their voices with messages which cause their current and would be allies and protectors to excommunicate and exile them (and their mentors) so that they can be "quarantined" and "detoxified". At this stage I will leave others to consider what those actions might be and how effective they would be - but many might regard Media and Internet censorship as preferable to the risk of mounting pressure for "ethnic/religious cleansing" after a series of copycat killings and reprisals, at all.
Meanwhile the Guardian carries an article on how propaganda techniques are being used in the Corporate world to drown our criticism, in parallel with an article that begins "The killers got their bloody hands on the front page first, but they struggled to keep the media's attention" - as though the target audiences the terrorists are seeking to reach are as flibbertigibbet as the media or share the values of the average Guardian reader.
The ancient Greeks probably had the right approach when they sought to ban the use of the name of the man who burned down the Temple of Artemis in order to be famous . They ultimately failed because the historian Theopompus recorded his name - but their approach may well have detered a generation or two copycat arsonists.
I suspect than one effect of this incident will be to accelerate the rate at which we move into an age of increasing Internet censorship and propaganda. I only hope that "our" side will be more effective than "their" side. I also hope that the funds available for cybersecurity are spend on that which is effective and relevant to the world of today and tomorrow - not that of the late 20th Century.