But Labour’s plans to restructure the economy represent a break with a neoliberal consensus of the past 30 years. This shift does have popular support, yet it might conceivably face institutional resistance. Britain’s civil service, which has met with the Labour leadership and which is democratically committed to political neutrality, could respond to populist left policies with a technocratic disposition toward continuity and slow, incremental change. While shadow ministers, some of them relatively inexperienced, are being prepped in working the levers of government so as not to get bogged down by bureaucracy, Labour advisers say they are looking toward countries such as Denmark or Germany for advice on implementing progressive policies.

But a key emollient in all this needs to be popular buy-in. Mr. Corbyn’s stated intent to democratize Labour is already taking shape. The parliamentary party is now in a feedback loop with its grass-roots: unleashing thousands of enthusiastic campaigners at street level, while declaring that “another world is possible” from the party leader’s podium.

In Brighton, over a thousand delegates — many of them newly elected — decided which issues would be debated and saw measures that they approved on one day appear in the leader’s speech on the next. Mr. McDonnell suggested key policies such as re-nationalization could be planned in consultation with trade unions, civil society, consumers and local authorities — providing not just extra accountability, but also another layer of legitimacy.

Of course, none of this straightforward and expectations should be managed in preparation for the long haul. Labour’s conference, while jubilant, did not feel complacent, though that is something to guard against. So, too, will be the pressure on Labour to tone down the platform that has made it so popular in the first place. Meanwhile, the regressive forces that helped push the Brexit vote have not vanished and could be reanimated by the right.

Still, if Labour’s policies are, as Mr. Corbyn says, now “mainstream,” if the center ground has shifted, if the unfettered free market has at last been publicly nailed as the source of division and hardship, and if the Conservatives keep free-falling — well, then it looks like the British left, after decades in the fringes, may finally have its moment.