When Donald Trump is inaugurated as the 45th President of the United States, the ceremony will be the culmination of a democratic process that is played out, come rain, come shine, every four years. Many criticisms can be made of the workings of US democracy, from voter registration to the gerrymandering of constituencies to the role of money and – as seen in 2000 – the actual mechanics of voting and counting.

Much harder to fault is the Constitution which underpins US democracy. Not only does it provide for the peaceful handover of power and a new president whose legitimacy – even after this bizarre election year – is largely unchallenged. It also enshrines the institutional checks and balances that constrain presidential power.

The US constitution is brief, to the point, and practical. Written more than two centuries ago, it has held up so well that many of us journalists covering the aftermath of the “tied” 2000 election took to carrying around a copy to find out what was supposed to – and indeed did – happen next.

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A written constitution is not everything. The juxtaposition between the orderly preparations in progress in Washington DC and what is currently happening in The Gambia, where the electorally defeated president is refusing to go, shows that respect for the constitution is a crucial element in what makes it work. But at least the existence of a document promotes clarity about what is and is not acceptable and supplies a blueprint for how the system should work.

In the UK we have prided ourselves on our “unwritten” constitution, made up of a body of legislation passed and amended over centuries. The idea that it is continually evolving to suit new circumstances is regarded as a strength, as is the way the UK public has accepted an arrangement in which rights, responsibilities and the limits of administrative power are scattered around many laws – if they are written down at all – rather than measurable against a single document such as a Constitution or Basic Law that sets the yardstick.

Increasingly, though, this prime source of national pride has been looking, if not obsolete, then inadequate to the modern world. Not having a category for defining something as constitutional or not is proving a liability in these legalistic times. Worse still, it turns out that, if there was any consensus about where Royal power stops and the sovereignty of Parliament begins, there is not any more – and probably has not been for some time.

This fundamental question is the one that will be decided, in part at least, by the Supreme Court next week, when it hands down its judgement in the case of R (on the application of Miller and Dos Santos) v Secretary of State for Exiting the European Union – the case better known as the one that will determine whether the Prime Minister or Parliament has the authority to invoke Article 50 and set the process of our withdrawal from the European Union in train.

Perversely, when such a high principle is at stake, the ruling will have little practical effect. If the court rules that Parliament must vote, it is confidently expected that MPs will respect the result of the referendum. So the idea that the legal challenge, if upheld, can strike a real blow against Brexit, is not actually so. But the principle, which boils down to how much remains of the monarch’s residual power, and whether EU membership can be revoked like a treaty with a foreign country – as the Government maintains – or whether, as Gina Miller and her legal team have argued, it is much, much more.

Brexit reactions – in pictures Show all 10 1 /10 Brexit reactions – in pictures Brexit reactions – in pictures Supporters of the Stronger In campaign look at their phones after hearing results in the EU referendum at London's Royal Festival Hall AP Brexit reactions – in pictures Leave supporters cheer results at a Leave.eu party after polling stations closed in the Referendum on the European Union in London Reuters Brexit reactions – in pictures Mr Cameron announces his resignation to supporters Getty Brexit reactions – in pictures Donald Tusk proposes that the 27 remaining EU member states ‘start a wider reflection on the future of our union’ Getty Brexit reactions – in pictures Ukip leader Nigel Farage greets his supporters on College Green in Westminster, after Britain voted to leave the European Union PA Brexit reactions – in pictures Supporters of the Stronger In Campaign react as referendum results are announced today Getty Brexit reactions – in pictures Boris Johnson leaves his home today to discover a crowd of waiting journalists and police officers Getty Brexit reactions – in pictures Leave EU supporters celebrate as they watch the British EU Referendum results being televised at Millbank Tower in London Rex Brexit reactions – in pictures Supporters of the Stronger In Campaign react as results of the EU referendum are announced at the Royal Festival Hall Reuters Brexit reactions – in pictures Supporters of the Stronger In campaign react after hearing results in the EU referendum at London's Royal Festival Hall PA

That a question which is at once so basic and so simple is not already clear in law surely illustrates one of the yawning gaps that our ever-evolving constitution has left. The place of referendums – their use, validity and form – should be another. Why, for instance, was no “constitutional” majority – say two-thirds – required to validate what was so obviously a constitutional issue (whether to leave the EU)?

But there is also an upside of the court challenge to the Government about Parliament and Brexit, which is that at least some of the uncertainties arising from our unwritten constitution have been laid bare. Not only that, but the most eminent lawyers in the land, both advocates and judges, have been involved in arguing – and are now ruling on – a constitutional case.

Constitutional matters are not going away. The next questions could very well be posed by another Scottish referendum. It could be argued, too, that it is a sad reflection on our national lack of constitutional sensitivity that such radical decisions as UK devolution and the creation of the Supreme Court – both of which have upset longstanding equilibriums – were not subject to judicial challenge.