Willie Whitelaw became prime minister in a predictable way, in 1979. He and his opponent in that general election, in retrospect, seem fairly indistingishable, avuncular presences. The Labour leader, James Callaghan, had grown familiar to the British public in a succession of roles – the only politician in history to have held all four great offices of state, having been chancellor, foreign secretary, home secretary and, from 1976, prime minister. His government had not been a great success; its worst economic moments were behind it, but by 1979 was collapsing in a welter of industrial action which had nearly brought the country to a standstill over the winter. His government was finally brought down by a vote of no confidence in the House of Commons. Would the leader of the opposition provide a more confident government? He certainly looked very much the same – another patrician, smooth, confident man. He seemed like a safe pair of hands, unlikely to institute any radical change. He was perhaps best known for remarking, during the 1974 election, that Labour had been "going round the country stirring up apathy". Whitelaw seemed like quite a reliable alternative. He wouldn't do anything to startle the horses, and in the end, after a well-fought, gentlemanly campaign, the country went for the Tories. The prices and incomes policies would keep the country going for some time to come.

To try to imagine what the country would have looked like without the dominant politician of the past 60 years is a dizzying exercise. Margaret Thatcher made the weather, and although serious analysts try to avoid ascribing too much to individuals, in this case it is as hard to imagine someone else carrying out the job of reconstruction as it is to imagine someone other than Napoleon directing the French forces at the Battle of Waterloo. There were other politicians around, thinking the unthinkable in the 1970s. Keith Joseph was starting to begin speeches by saying that he wanted "to put the moral case for capitalism"; Enoch Powell was a prophet of free-market economics. There were some extraordinary, sometimes borderline unbalanced journalists and theorists of the unfettered market, railing against the dirigiste tendencies of the time. None of them could have begun to do what Thatcher did. Joseph and Powell were apt to think out loud to disquieting effect, pondering what would happen if the lower classes continued to breed at such a rate or immigration were not halted immediately. They wouldn't do as leaders of the party. Nor were the theorists and activists at all easy to imagine on the stump. Even those who were more presentable could not, surely, be relied upon to show the same steadiness of purpose, bordering on monomania, that Thatcher found indispensable. They would have U-turned as Heath did on economic policy, 10 years before. No: only Thatcher could have done it. What would have happened without her?

Given that Whitelaw could have allowed himself to replace Heath in 1975 or 76, a government of his would have been just as likely to win over Callaghan's – the country was sick of the corruption, chaos and indirection, and the vote was a negative one away from Labour. The opposition would, just as before, have elected Michael Foot as leader in 1980; in reaction, the Gang of Four would have split off, just as really happened, to form the SDP in 1981.

In Thatcherless Britain – would the mines still be open? Photograph: John Downing/Getty Images

A Whitelaw government would, overall, have continued the interventionist economic policies of the previous decade. It is possible to imagine that it might have pursued some of the privatisations that Heath had begun – for instance, Heath had privatised the travel company Thomas Cook in 1972, and the pubs in Carlisle, which had been owned by the state, in 1973. A Whitelaw government might also have gone on to privatise the removal company Pickford's, though perhaps not much further. The Post Office would have continued to supply telephone services, at its leisure. But in general, there is no reason to suppose that Whitelaw's government would not have maintained prices and incomes policy and exchange controls. The Prices Commission would have continued to control what customers could be asked to pay. Exchange controls would have continued, restricting the amount of money even tourists could take out of the country on holiday. The top rate of income tax might well have remained at its pre-election level of 83% on earned income, and the basic rate at 33%. This would have been the politics of economic consensus, and the ongoing commitment to full employment rather than controlling the money supply would have led to economic catastrophe with no way out.

A Whitelaw government would have differed from the opposition through its imposition of "short, sharp, shock" penal measures – as in reality, they didn't work, were expensive, and would still have contributed to the breakdown in law and order in the summer of 1981. Consensus politics would have led to deals with miners, steelworkers, British Leyland, the print unions. A steady decline in foreign investment would be matched by a year-on-year increase in days lost to industrial action. The final straw, as so often, would have been symbolically rather than substantively damaging; when Galtieri invaded the Falklands, would a consensus-led government have seen any point in fighting for them? Would this quixotic and absurd task have seemed other than unaffordable to a government of chums, and its avoidance other than humiliating to the electorate? An unpopular, ineffective, expensive, failing government would have been nothing new in 1983 – indeed, a new idea that UK governments reliably failed after four years would be the orthodoxy in political theory. A Labour government under Foot would take office.

We know what the 1983 Labour government would have done. Its manifesto, A New Hope for Britain, is quite clear. It would have embarked on unilateral nuclear disarmament; it would have withdrawn from the EEC; it would have abolished the House of Lords; it would have renationalised anything that had passed into private hands, perhaps adding the pubs of Manchester to the pubs of Carlisle, and going on to acquire Sainsbury's, Dolcis, the Daily Mirror, Knight Frank & Rutley and the Midland Bank. There would have been no question about income tax, which would have remained at high levels – perhaps the top level would even have increased to the 98% charged on unearned income. Perhaps unearned income, and the capital from which it derived, would have been confiscated in its entirety. It is not incompatible with the urges visible in the 1983 Labour manifesto.

In Thatcherless Britain – Michael Foot for PM? Photograph: Don McPhee for the Guardian

The Britain of this 1986 is not necessarily a very cheerful place. You are employed by the government, whatever you do, and your pay is set by a central body, matching driving instructors' pay to shop assistants' to filing clerks' to journalists'. If you want to go out for dinner, there are a few restaurants in Soho serving Serbian specialities, thanks to our new friends, on the three days a week when there is electricity to eat by. What Britons pay for the single brand of washing powder in the state-owned supermarket is determined by another central body. It is a long time since anyone has been abroad on holiday, or met an American or an Arab – anyway, a new passport takes a year to arrive. On the other hand, that ubiquitous figure, the Marquis of Headington (not so long ago, Robert Maxwell) has been forging strong links with his old friends in eastern Europe. President Ceausescu recently came to pay a visit. There are other international alliances, after all, than the EEC, which we left, and Nato, which we are leaving. Soon, there will be other avuncular figures arriving at the airport to shake the hands of Foot, the Queen and Lord Headington: visitors from the Kremlin with fur hats and bushy eyebrows, full of detailed advice, in no particular hurry to leave.

Possibly: or just as possibly, the country would have muddled through, getting by with one government or another, not doing terribly well, managing decline. It could have been Portugal. There was a historical-necessity aspect to Thatcher, by which I mean that anyone, sooner or later, would have perceived that full employment wasn't achievable without terrible costs elsewhere, and that there was no reason for the state to own removal companies, and that an 83% top rate in income tax might not be the best way to encourage enterprise and industry. Those insights didn't need Thatcher. There was also a Thatcher aspect to Thatcher, and some of what she did had to be done by her, for good or bad. Who else would have seen an end to the cold war from the start, saying at the Berlin Wall in October 1982, "One day they will be free", when every other western politician would have urged tact? Who else would have decided to embark on a war not just against miners' unions, but against miners in a spirit of revenge for which she will never be forgiven?

By now, we would probably be roughly where we are. Surely, someone else would have made the reforms, or some of them, anyway. But it would not have happened in exactly the same way, and some of it would not have happened at all. Perhaps we would be waiting six months for a mobile telephone, and paying the bills to the post office, headed by the Postmaster General – I don't believe it would be a very advanced telephone, either. Perhaps there would be three TV channels and the requirement for a licence before you could use the internet.

Some people see possibilities, and transform the world for good or ill; most of us rely on what has gone before, and perpetuate the ordinary. If Thatcher had never lived, some of her revolution would have taken place anyway. It had to. All of it? No. What you think of that revolution is entirely up to you.