It turns out that there is a nuclear button that Theresa May is reluctant to press. She has just made her first big policy decision by choosing to pause and review government support for the mammothly controversial Hinkley Point nuclear reactor. Be in no doubt that this was a call made in Downing Street. It may have been Greg Clark, the business and energy secretary, who released the surprise news in a terse, late-hour statement from his department, but the order to delay came from Number 10. Only days earlier, the chancellor, Philip Hammond, had told Kamal Ahmed, the BBC’s economics editor, that the plant was definitely going ahead.

The pause has evidently come as a complete shock to EDF, which had already ordered in the champagne for the contract signing after the board of the French state-owned company signed off on the project on Thursday.

Even if it ultimately gets the green light in the autumn, choosing to reappraise the Hinkley C project is a significant act in itself. The pause has cheered and emboldened enemies of the plant, both those who are against it on the grounds that it is a vanity project that will prove ruinously expensive to the taxpayer and those who are opposed to nuclear reactors because they involve splitting uranium atoms.

The delay has frustrated and angered the project’s supporters who ask how this fits with the government’s attempt to convince the world that a Brexit Britain will still be open for foreign investment. It has added tension to relations with the French at a time when they are already strained by Britain’s vote to leave the EU. The Chinese, to whom face matters so very much, aren’t pleased that their heavy involvement was clearly one of Mrs May’s problems with the project. The GMB, the relevant trade union, has called the delay “bonkers”. Labour people have had an opportunity to describe something other than their own party as “chaotic”.

This decision has also shed some revelatory light on how Mrs May intends to operate. She wants her Number 10 to call the big shots – and she wants people to know that it is calling those shots. Another thing it underlines – and this point is so obvious that it has been almost completely missed – is that Mrs May has not been prime minister for terribly long. She moved into Downing Street less than three weeks ago. She had less time to prepare for the transition than initially expected because it was accelerated when she received the unopposed coronation that she originally said she didn’t want.

Since Mrs May acquired the keys to Number 10, she has not spent all that much time in her new home. Many of her early days as prime minister have been spent travelling around Europe – if it’s Thursday, it must be Warsaw – trying to get acquainted with leaders who will be crucial to the Brexit negotiations. She has also devoted time to visits to Scotland, Wales and Northern Ireland for Brexit-related reasons. I’m told that Mrs May is already finding that Brexit discussions, both the talks with European leaders and those taking place within her cabinet as ministers try to thrash out an agreed negotiating strategy, are consuming much more of her diary than she hoped.

That has left her with less time to address an in-tray groaning with a pile of other decisions waiting to be made. This bequest of unfinished business from her predecessor did not accumulate because David Cameron was a chronically indecisive leader. It was often a better critique of him that he made major calls without spending sufficient time thinking through all the consequences. The backlog of decisions has mounted up because the energies of government have been so consumed by Europe for the best part of the last year: first by the pre-referendum negotiations, then by the long referendum campaign, and in the past month by the political turbulence and economic uncertainty unleashed by the result.

In those breathing spaces that have not been dominated by Brexit, Mrs May has been endeavouring to immerse herself in a wide array of subjects with which she had only a glancing engagement during her six years as home secretary. She was party to earlier cabinet discussions about Hinkley Point, but these were largely superficial. Cabinet long since ceased to be a forum for meaty policy debate and crunchy decision-making, not least because prime ministers have become so fearful of discussions being leaked. Were Mrs May of a different temperament, I suppose she might have been content to devolve the decision about Hinkley. But the other important players are also new to their jobs and, anyway, we know that Mrs May is not an instinctive delegator. She is also usually at the careful rather than the rapid end of the spectrum when it comes to decision-making. Impetuosity is not her hallmark. She doesn’t like to be rushed or bounced. Her preference is to work through problem issues by reading all the relevant papers and keeping discussion tight with the small group of people whom she really trusts. Colleagues report that she hates having to commit to a course before she has examined all the angles and the Hinkley project is freighted with a lot of risks. The near-horizon cost is £18bn and rising. The potential future charge on taxpayers and electricity bill payers is higher still. Britain has had a disillusioning history when it comes to nuclear power, a technology this country largely invented but has been very poor at using successfully. If Mrs May wants to do further reading on the subject, I recommend Professor Simon Taylor’s excellent recent book, The Fall and Rise of Nuclear Power in Britain. When so much money is involved, and so many people think there is such potential for this project to go very badly wrong, it is not really so astonishing that Mrs May wanted a pause.

As a new prime minister with a lot on her plate, there is some willingness to forgive the chaotic manner in which the review was announced, but she won’t always enjoy the benefit of the doubt. A previous occupant of her job once told me that Number 10 has to make about a hundred decisions a day of one sort or another. Many of those decisions are big. A lot of them are urgent. It remains to be seen whether Mrs May’s cautious method will be entirely compatible with the sheer velocity of decision-making that is required of a modern prime minister operating in a 24/7 media world.

Facebook Twitter Pinterest Nick Timothy, the prime minister’s joint chief of staff, has long opposed the deal with China. Photograph: Dominic Lipinski/PA

One thing she does know a lot about from her previous role is security. “There will always be a little bit of the Home Office inside me,” she told her civil servants when she made her farewell speech at Marsham Street before moving to Number 10. The Hinkley deal binds Britain into a high level of Chinese involvement in other nuclear reactors and that has always troubled her on security grounds. One of the fiercest critics of making China so central to critical infrastructure and energy supply has been Nick Timothy, a long-standing aide of the prime minister who is now a figure of considerable influence as her joint chief of staff. He usefully put a lot of his thoughts about big issues into the public domain during a period out of government when he shared his views with the ConservativeHome website. In one post, Mrs May’s right-hand man accused David Cameron’s government of “selling our national security to China” on the grounds that “the Chinese could use their role to build weaknesses into computer systems, which will allow them to shut down Britain’s energy production at will”.

It is a big black mark for Hinkley in the mind of Mrs May that the project is an inheritance from her predecessor. Since she arrived at Downing Street, she has made a conscious effort to project herself not as a continuity with Mr Cameron, but a break from him. Her early statements as prime minister have contained a lot of implicit disparaging – and some explicit criticism – of his record. His allies were ruthlessly culled in her sweeping reshuffle. Her working majority in the Commons is 17. That is smaller than the number of ministers she sacked, which is likely to become a problem when her honeymoon fades and festering grievances surface. It wasn’t enough for her to dispatch George Osborne. Her people let the world know that he had been fired as chancellor rather than allow him to preserve some dignity by dressing it up as a resignation.

The new regime at Number 10 instinctively treats any legacy from the Cameron-Osborne government as a dodgy bequest of the most suspect provenance. It was the reverse of helpful for the Hinkley deal that its most eager sponsor in the previous cabinet was Mr Osborne. That in itself made it suspect in the eyes of Mrs May. The former chancellor was also the keenest champion of a third runway at Heathrow so I would not be betting on that happening. She has already junked his deficit reduction plan and downgraded his focus on the Northern Powerhouse.

It will take a while before we get the full measure of how Mrs May will perform as prime minister. But we now have a useful, early rule of thumb for predicting how she will act. Imagine what George Osborne would do in any given situation and assume that Theresa May will do the opposite.