The Prime Minister got an angry reaction during a hospital visit this week when he told the father of a sick child there were “no press” at the event – while being filmed by a BBC camera crew.

Boris Johnson was widely accused by critics of telling an obvious lie to the father, who later identified himself on social media as a “Labour activist”.

It’s possible that Mr Johnson had something else in mind when he implied the people filming were not “press” – and this gives us an insight into how politicians try to manage the media.

What happened?

Omar Salem was waiting for his daughter to be treated in a paediatric ward in Whipps Cross hospital in northeast London when he confronted the Prime Minister, saying: “The NHS is being destroyed… it’s being destroyed and now you come here for a press opportunity.”

Mr Johnson replied: “Well actually, there’s no press here.”

Gesticulating at the members of the media standing nearby, Mr Salem said: “What do you mean, there’s no press here? Who are these people?”

Mr Johnson appeared to start an attempted explanation, saying: “They’re… they’re here…” before trailing off.

Who was at the event?

Downing Street invited a photographer and a video crew from the Press Association – the UK’s news agency of record – to the event, as well as a “pool” video crew, consisting of a ​camera operator and broadcast journalist.

The long-established pool system involves the major broadcasters (including ITN, the makers of Channel 4 News) taking it in turns to send cameras and journalists to events that we know about in advance.

Footage shot by the pool camera crew is then made available to other members of the pool for use in their own broadcasts.

Often, the pool team agrees to go to the event on the understanding that a reporter will get the chance to ask questions of politicians like Boris Johnson.

Obviously, if only one reporter attends, that limits the scope of the questions, but the questions aren’t pre-arranged with the government and there’s no restriction on what journalists can ask.

On this occasion, the broadcasters who run the pool were told that they would be able to film Mr Johnson walking around the hospital visit, but no arrangements were made to record an interview the Prime Minister, or ask him questions.

The pool team agreed to go on this understanding. Apart from the Press Association, it appears that no other reporters from newspapers or other news organisations attended.

This may have been what Mr Johnson had in mind when he said there were “no press” at the event.

While there were people filming him walking the corridors of Whipps Cross, there were no reporters waiting to ask him awkward questions about the NHS.

How much control did Downing Street have over the hospital visit?

The pool system involves a certain level of co-operation between the government and media outlets on planning.

The idea is to ensure journalists get access to set-piece events without all the broadcasters sending teams of people to the same event to get the same shot.

That would be a waste of resources and a potential logistical nightmare for everyone. Imagine if half a dozen different camera crews had been pushing and shoving in the corridor of a busy hospital while doctors and nurses were trying to treat patients.​

Pool crews are often given the chance to ask questions, but not always. On this occasion the pool agreed to send people to cover the event without the understanding that a reporter would get to question the Prime Minister in an interview.

Obviously, the government doesn’t have any say in what other footage is filmed and broadcast.

On this occasion, a potentially awkward exchange in the hospital corridor was caught on camera and quickly put on air.  No one from Mr Johnson’s team had the opportunity to interfere with that.

So there is clearly a limit to how much control the government has over how these photo opportunities play out.

On the other hand, it appears that Boris Johnson was effectively able to avoid having journalists asking him difficult questions on this occasion, by limiting the number of invitations sent to news organisations, and by not offering anyone the chance to film an interview.

This comes amid wider concerns from broadcasters about the access they get to senior politicians.

Channel 4 News has not been granted an in-depth interview with either Boris Johnson or the Labour leader Jeremy Corbyn despite repeated requests, and BBC Radio 4’s Today programme has made the same complaint.