We always thought that policing by consent was the British way. Our constables were simply citizens in uniform. Not for us the weapons, masks and boots of the continental riot police and their political masters. If you wanted a difference between us and the rest, this was it: no policeman in Britain would ever go about his business with his face covered.

This was a style of policing that minimised the risk of violence and abuse. It was a philosophy that, to this day, keeps the majority of our officers from bearing firearms. It is an approach that has overwhelming public support.

It is also an approach that has sometimes been undermined in practice. Yet the disgraceful conduct of the Special Patrol Group in Southall in 1979, those out-of-area officers waving their £20 notes in mining villages in the early 1980s, even the scandalous organised perjury of the West Midlands Serious Crime Squad could not completely destroy a bond between people and law enforcement in this country. It remained a defining characteristic of our public life and Britons were very proud of it.

And it survived despite the fact that identifying wrongdoers in the police service has always been challenging. Speaking of the death by brain injury of young schoolteacher Blair Peach in the Southall riots, Lord Denning famously said there was much evidence he had been killed by a police officer - and no evidence he was killed by anyone else.

But the assailant was never produced. And when illegal weapons, including coshes and knives, were found in its lockers, the SPG was ignominiously disbanded. The Metropolitan police commissioner, Sir Paul Stephenson, a thoughtful and honest man, will be anxious that its successor, the Territorial Support Group, is acquiring a similarly unenviable reputation.

It is not systematic corruption or even political compromise that most threatens the relationship between the British and their police in this new century. In a deeply ironic reflection of the government's obsession with surveillance, it is the newly discovered power of mass photojournalism.

Now a single punch, or a glancing whack of the truncheon can be captured and broadcast to the watching world without control. It seems anyone with a camera can do a better job than the Independent Police Complaints Commission. Indeed, it is apparently only those people with cameras who can prise a decent distance between that body and the police they are supposed to be watching.

The IPCC will come under severe strain if it repeatedly finds its instincts belied by the jumpy, inexpert, but deeply telling frames of evidence that jolt out at us from the evening news. It is a signal of the IPCC's struggle to establish a robust reputation for independence that no one can imagine this material being sent directly to its own investigators rather than to journalists.

The identity of police officers matters. That is why they all have numbers. As we citizens are sometimes lectured by home secretaries: if you have done nothing wrong, you have nothing to fear. In the early 1980s, after an innocent young black man leaving a funfair in north London was badly beaten in the back of a police van, it was seriously contended by the Metropolitan police that the vehicle in question could not be identified. Subsequently, all police carriers had numbers painted on their sides.

We need to be on the same side as the police. And the police need to be on the same side as the rule of law. In classical common law doctrine, this means that they are subject to exactly the same constraints as the rest of us. This is not a weakness in their armoury. On the contrary, under our system it is their greatest strength because it brings the police the co-operation and consent of the public.

So here are some questions for the IPCC to consider as it investigates the events leading to Ian Tomlinson's death: why were British police officers attending a demonstration in the heart of London with their identifying numbers hidden? In the absence of a fire risk, who authorised them to pull balaclavas up over their heads? And why didn't they want anyone to see their faces?

• Ken Macdonald QC practises from Matrix Chambers and was director of public prosecutions, 2003-2008