LET'S be clear: the Scottish Parliament can't block the Not-So-Great Repeal Bill that was presented to Parliament last week. Yes, the EU (Withdrawal) Act is an assault on devolution, as I will explain, but Holyrood has always been constitutionally subordinate to Westminster and only exercises legislative power with Westminster's authority. “Sovereignty”, as the 1998 Scotland Act avers, “remains with Westminster in all cases”.

But that isn't the end of the matter. Holyrood does have the right to pass, or refuse, a legislative consent motion on the EU (Withdrawal) Act. The UK Supreme Court has said that this is “a political convention”, but constitutional conventions aren't just for show. They do mean something. For example, it is only a convention – the “Salisbury Convention” – that the unelected House of Lords is not able to reject legislation that was in a governing party's manifesto before they were elected.

Similarly, the Sewel Convention, which says that the devolved parliaments have to give consent to any changes in their powers, is more than just a form of words. Holyrood's powers may be subordinate, but that doesn't mean they don't exist. It is a constitutional offence for the Withdrawal Bill to say nothing of this, and simply assert that all powers repatriated from Brussels will go, in the first instance, to Westminster. This is a new and controversial interpretation of the law – one that Wales, certainly, is determined to challenge.

Indeed, it was rather strange last week hearing the Labour First Minister of Wales being more nationalistic, in one sense of the word, than Nicola Sturgeon. Carwyn Jones has not only threatened to withhold legislative consent from this “naked power grab”, he has suggested passing a Continuity Bill in the Welsh Parliament. This would import EU powers over matters like agriculture, the environment and consumer rights, directly to Cardiff, rather than allow them to go to Westminster. This was dismissed as idle troublemaking in UK Government circles, but it does make sense. If nothing else, it would test in the courts the extent to which the devolved parliaments do exert power autonomously.

If Nicola Sturgeon seems somewhat less resolute in her opposition to the repeal bill than Carwyn Jones, it may be because she is more of a realist. The SNP doesn't really believe in devolution, regarding it as at best a series of concessions from Westminster to head off independence. Sturgeon seems willing to bargain support for the repeal bill against a seat “at the Brexit negotiating table”, or for more powers. I think Nicola Sturgeon broadly believes the Scottish Secretary, David Mundell, when he says that there will be more powers for Holyrood (eventually) in areas like the environment, consumer rights and energy. She is in the business of getting the best deal available.

But the Welsh really believe in devolution, and regard Cardiff's law-making powers as constitutionally sacred and not to be tossed about at the whim of UK ministers. Like Sturgeon, Carwyn Jones accepts that there will have to be agreed UK frameworks for matters like agriculture, if only to ensure that there aren't customs duties between Wales and England. There has to be a new internal market, with its own rules, to replace the EU single market that Britain is leaving. However, what Jones is not prepared to accept is legislation which effectively extinguishes the devolved parliaments as stakeholders in this process.

Part 3 (2) of Schedule Two of the EU Withdrawal Bill is very blunt. It asserts that the devolved parliaments cannot amend the bill, or make changes that would be “inconsistent” with those made by Theresa May. This means that there is no scope for Holyrood to keep EU standards for workers' rights, insulation in buildings, genetically modified crops, hormone-fed beef. Carwyn Jones is right to call this a “power grab” because it excludes any role for the devolved parliaments except as passive recipients of powers which may later be granted by the UK Government.

Once Theresa May has sifted through the 19,000 regulations, directives and other measures that are being imported into UK law, she is at liberty to change any and all of them. Only after this process has been completed does the question of Holyrood's powers emerge. At the very least, there should have been some idea of co-decision-making – as there is in the EU – between the devolved parliaments and Westminster.

It's not hard to see why the UK Government is behaving in this way. Time is short and the Brexit road is difficult enough without allowing the devolved parliaments to put their oars in – or to “sabotage Brexit”, as the UK tabloid newspapers put it. The repeal bill gives Theresa May Henry VIII powers to bypass Parliament on decisions about which EU laws to keep and which to discard. Labour is promising to oppose her refusal to import the EU Charter of Human Rights into UK law and many of her own backbenchers want the UK to stick with the EU body, Euratom, which oversees nuclear safety.

She has been assured by her legal advisers that Westminster can overturn any Sewel Motions from the devolved parliaments. However, it may not be as easy as that. If Wales and Scotland dig their heels in over issues like the environment or food safety, it may be that MPs in Westminster will be reluctant to overturn the legislative consent motions. This might have been more likely had the SNP leader not lost authority following the General Election. No-one in Westminster thinks that Nicola Sturgeon is going to call another independence referendum. But the moral case might still carry weight. And remember that Theresa May is beholden to a Northern Irish party, the DUP, which is pro-Union, but can't afford to see devolution exposed as a sham.

For all the high-flown rhetoric of the EU Withdrawal Bill, the future of Brexit is currently on a knife edge. Theresa May has lost her parliamentary majority, there is discontent on her back-benches, Labour is increasingly assertive. The negotiations are in chaos, with the Foreign Secretary, Boris Johnson, saying one day that the EU can “whistle” for any big divorce settlement only to be contradicted the next by the Brexit Secretary, David Davis, who says that Britain will continue to pay into the EU budget for many years. The Chancellor, Philip Hammond, under pressure from business, is angling for a transition period in which Britain will remain in the single market.

The UK establishment is in chaos and increasingly fearful that a cliff-edge Brexit could actually happen with profound economic consequences. On Friday the London Times suggested that Britain should opt for an interim relationship “similar to that enjoyed by Norway”. Intriguingly, this is very similar to Nicola Sturgeon's position and that of Carwyn Jones. If the devolved governments play their legislative cards right here, and argue their case (hopefully with the support of opposition parties both in Holyrood and Westminster) then they could help deliver a soft Brexit. And prevent the legislative autonomy of the Welsh and Scottish Parliaments being extinguished by ill-thought-out legislation that places arbitrary power in the hands of the UK Prime Minister.