“I’m not a deeply ideological person,” David Cameron told the Observer in the days after becoming leader. “I’m a practical person, and pragmatic.” In this he really does honour the “heir to Blair” tagline – the only one that has ever stuck.

With dramatic timing, taking the country to war in Syria will coincide with the prime minister’s 10th anniversary as Conservative leader. On 6 December 2005 he became one of the youngest people to take over the oldest political party in the world. In a party famed for the managed regicide of its leaders, it makes him only the fourth person in a century to reach this milestone. In other words, he has achieved what the likes of Macmillan and Major and Chamberlain could not accomplish before him. He has survived.

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I wrote Cameron as a character in my Channel 4 drama Coalition. Mark Dexter, who played him (and again on stage in Peter Morgan’s The Audience), told me that “by the end of these projects I think I’d got as deeply under Cameron’s skin as any actor could get. But in spite of this, I ended up absolutely clueless about what motivates him, or about what Dave wants.”

The difference between Cameron and his titanic predecessors is that he has almost completely managed to escape definition. There are no “Cameronites” flying the flag of “Cameronism”. Supporters say that this is one of the reasons he has endured.

But regardless of where you stand, his mission – and it is a mission, and all missions are ideological – has been a shift in the powers and perception of the state that would have made Margaret Thatcher, subject of the definitive “ism”, gasp. His agenda has been so palpably denied an ideological tag because of the narrative Cameron and his coterie have spun: that they were simply reacting sensibly to the conditions they inherited, and had no choice. They may very well believe that. The only problem is – it isn’t true.

So, for a man who has so successfully spent his life avoiding litmus tests and labels – how then are we to judge him?

He walked into his audition 10 years ago, right out of the blue, with a “compassionate Conservatism” conference speech that left David Davis, his main rival for the leadership, unable to recover. His pitch, long before Barack Obama, was change. “We will change the way we look … We will change the way we feel … We will change the way we think.”

Facebook Twitter Pinterest ‘Apart from the hugging of the hoodies, PR man Web Cam struggled to find his unique selling proposition.’ Photograph: Martin Rickett/PA

Just words, the Tory grandees must have thought. But Cameron wasn’t kidding, as the old guard were handed an actual fruit smoothie and ordered to stop being so mean to poor people and foreigners (for now). In his victory speech he announced his core philosophy that everyone seems so determined to convince us he doesn’t have. “There is such a thing as society. It’s just not the same thing as the state.”

And yet, apart from the riding of the huskies and the hugging of the hoodies and the “greenest ever” promises long since forgotten, PR man Web Cam struggled, for a while at least, to find his unique selling proposition. He and his tight political conclave even vowed to match Labour’s spending plans (much as they’d like us to forget). Cameron readily admits that had Gordon Brown not “bottled out” of going to the country in 2007, he would have lost any election and that would have been that.

And then came the financial crash, and a once-in-a-generation opportunity.

The success of his “austerity is the only option” narrative is impressive, but cutting on this scale was not considered inevitable by other countries. A 2010 editorial in the New York Times (not known for anti-market fanaticism) said: “Prime minister David Cameron’s government … could suffocate a feeble recovery. Unfortunately, Britain’s leaders chose posture over economics.” And so it proved, with the national debt rising every year under Cameron, despite deep sacrifices from us all.

But the debt was only ever half the point. Cameronism may be in thrall to Thatcherism, but its roots go back to a 19th century view of welfare and society. The England he grew up in entrenched these views: an England of parish volunteers, cricket and hunting. It was an idyllic childhood that rarely came in contact with the state. At Heatherdown prep school he performed with fellow pupil Prince Edward, and the Queen came to watch.

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Yet for all this status quo, Cameron is still the modern, more socially liberal Tory who pushed through gay marriage against the will of his party – his philosophy on freedom and choice makes him believe that people who love one another should have the same opportunity to express that love. I believe he’s right.

This is Cameron at his best. Cameron at his worst, and incidentally most distinctive, is when he loses his cool. The “calm down, dear” Cameron who also belittled the likes of Dennis Skinner in the house by mocking his age.

He is capable of statesman-like rhetoric, as with his impassioned plea before the independence referendum: “We want you to stay. Head and heart and soul, we want you to stay” – a nobility he immediately undermined, launching into the all-out revenge of English votes for English laws the morning after the vote.

And this week came the incredibly misjudged “terrorist sympathisers” slur against opponents of airstrikes in Syria. Yet these flare-ups are rare. He generally comes across as level-headed and sensible, which helps convince us he’s not ideological. After all, when we think of an ideologue we think of the inflexible stubbornness and passionate temper of Gordon Brown.

Cameron has an easygoing charm not entirely unrelated to the self-assurance that comes with his class. One of his most notable achievements is the way he has kept Nos 10 and 11 in a harmony unheard of in years: he and Osborne make sure they meet at least twice a day, in the morning and afternoon. Cameron impresses his civil servants by always getting to the bottom of his overnight red box – again in contrast to his contemporaries. His Downing Street is professional and functional. The people who work with him like him. He is described as “able”, and he is described as “nice”.

Yet history often judges leaders by the manner of their leaving: like Thatcher, under the cover of darkness to hide the knives in her back; or Blair, of his own volition in the light (sort of). Cameron has pinned his hopes on the latter.

His imprint on the Conservatives may not be as deep as the one he leaves on Labour – leaders are also defined by the type of opponent put up to vanquish them by their enemies. Just as Thatcher believed her greatest success to be Blair, Cameron may yet claim Jeremy Corbyn as his prize, and the belief that Labour will be out of office for the foreseeable future.

Regardless, the country he leaves before the next election will be vastly different to the one he set out to lead a decade ago. To his allies it is freer and more prosperous; to his opponents, more unkind and more unequal than ever. Either way, what is undeniable is the way he has shaped a Britain based on a clear “idea” of what he thinks it should be, while convincing people he doesn’t have one. A remarkable idea in itself. Even if it isn’t true.