Boris Johnson is too cynical a politician to allow the events in Streatham to pass without making some political capital out of it, particularly as the electorate will want answers anyway.

So it will be with the latest attempt to kill innocent people. Expect, then some of the usual tough sounding rhetoric in the coming days, promises of new laws and, maybe, even, some attempt to blame Labour for what happened. After all, that was precisely what happened when the Fishmongers Hall attacker, Usman Khan, was found, like this one, to have been a recently released prisoner. Khan killed three people on London Bridge before he was restrained and shot dead.

The Conservatives had been in power for almost a decade, yet somehow it was all spun by Johnson and Priti Patel as Labour and Jeremy Corbyn’s fault. It was not a great moment for British democracy.

The “obvious” answer, we are again being told, is to lock up anyone with even a trace of terrorist sympathies for ever longer spells. It is not a practical policy. It will not stop future attacks.

One of the few concrete promises in the Conservative manifesto is to “keep terrorists in prison for longer”. Such pledges were repeated after the last terror attack in London, along with some poorly-formed plan to use lie detectors to identify those who are concealing their violent instincts. If lie detectors are not good enough for the Jeremy Kyle show then it is not going to be sufficient to stop extremism.

The problem with all of this judicial action is that it won’t work, and will merely add to the public’s rightful indignation about the authorities’ ability to keep us safe. Delaying terror attacks is not the same as preventing them.

Keeping terrorists “in prison for longer” is not the same as indefinite detention. A life sentence cannot sensibly be imposed on someone for downloading from the web some Daesh propaganda or a knife attack manual or bomb making instructions. Extending their tariffs arbitrarily (for existing prisoners) and abolishing parole is problematic for obvious reasons.

No government should be able to simply pass a law to keep people in prison who have already been sentenced – otherwise why bother with courts? If the intention is to bang ‘em up for longer in future without a right to parole – so full tariffs are more often served – then the hope of rehabilitation is correspondingly diminished. Some believe that terrorists cannot be rehabilitated into society. Well, some cannot, but not every potential terrorist or convicted terrorist is beyond reason or redemption. Some have been deradicalised and are actively now fighting terrorism.

Just take a moment to peer into the mentality of Sudesh Amman and Usman Khan, the killers of Lee Rigby, the 7/7 and the Manchester Arena bombers and every other self-styled rebel; none are likely to be deterred by longer sentencing, with or without parole.

Changing laws and detaining people without trial has never extinguished terrorist movements. Even virtual police states suffer terror attacks with the most stringent controls, emergency laws and extensive precautions and draconian vengeful rules have failed to stop such violence.

The terrorist will sometimes get through, particularly in an age of low-tech terror, when all that is needed is a van or a kitchen knife. Shredding civil rights and the rule of law may make people feel more secure but it does not make them more secure. Boring intelligence gathering and boring community police work, and strategies such as Prevent (though flawed) are the most effective way of stopping people becoming terrorists in the first place.

There is a bitter irony in the fact that British citizens were injured virtually on the steps of the old Streatham police station, closed during the years of austerity when Johnson served as mayor of London.

Vital work was lost when 20,000 police posts were scrapped during Theresa May’s time as home secretary and prime minister. The chief secretary to the Treasury, Rishi Sunak, has denied that cuts to prison and probation services have made it harder to rehabilitate or monitor terrorism suspects – claiming that the counter terrorism budget has been increased.

But the question has to be asked. The last three Tory prime ministers, including David Cameron need to be held accountable for what they did, and did not do, to preserve the continuing safety of the British people.