In just a week, the debate about Scottish independence has shifted perceptibly. Full independence is not a likely outcome but then nor was an SNP majority in last week’s Scottish parliamentary elections. Certainly, a third way of fiscal independence, while remaining part of a monetary, defence, and foreign affairs union, is not unthinkable. 

However, this cannot be just a question for Scotland. It concerns the whole UK. It’s a question for England and Wales too (we’ll leave Northern Ireland to one side for now). We won’t get a vote – it is the Scot’s choice ultimately. We are most definitely part of the discussion though. It has enormous repercussions for England – and there are particular issues for those of us on the centre-left. These arguments emerged sharply for me after a stimulating informal scratch discussion hosted by IPPR and openDemocracy yesterday.

Both David Cameron and Labour have critical strategic decisions to make regarding how to approach the referendum question. Alex Salmond holds all the cards. He has to hold a referendum in the next five years but that’s his only limitation. Beyond that, he can hold it on the terms and timing of his choosing. He will be able to play Coalition austerity against his own resistance and resolution. He may decide to have three choices on the ballot – full independence, fiscal independence, and the status quo – with a preferential vote on which proceeds. That would be a good way to hedge. He’s calling the shots. He has everything to gain; fail, though, and his political mission comes to an abrupt end. 

Cameron’s choice is easy. He is a unionist but his opposition to any move towards the break-up of the UK will be more mission critical than that. Who would want to be the authority-sapped Prime Minister who had failed to even hold the country together? Whatever the English may think about the Scots (indifference in the main as things stand, though opinion is evenly split on independence as a new YouGov poll shows), they will be slighted by independence and Cameron would carry the can.

So there is likely to be a short-term political cost of independence for the Conservatives. Whatever arguments are made about the (highly disputed) money saved through independence – will we see an English Taxpayers’ Alliance type campaign in favour? – or about the reduction of Labour’s representative bonus, Cameron will fiercely resist.

Unfortunately, the long-term political costs are Labour’s or rather the centre-left’s. A centre-left majority in England and Wales will most definitely be the exception rather than the rule. Michael Howard even managed an English Tory majority in 2005 (though not in terms of seats). Those who hold social democratic, Green, social liberal, or non-Tory views of any kind potentially have much to lose from Scottish independence. However, English partisan interest will not weigh heavily in the Scottish referendum debate. Instead, Labour will need a Plan A and Plan B.

Plan A bears a striking resemblance to David Cameron’s Plan A: protect the Union. Alex Salmond will run an energetic, optimistic and forward-looking campaign for independence. The Scottish unionist parties and their English counterparts must do a similar thing in favour of union. Don’t let Alex Salmond own optimism. There will be an enormous price to pay for running a bitter and negative campaign against a popular and widely seen as competent SNP First Minister. Sunder Katwala suggests very sensibly sending Charles Kennedy to the Scottish Office as a kind of political expeditionary force. Jim Murphy used this platform to good effect.  

If the UK is to survive, Scotland must actively desire to remain a member. And so must England; any English chauvinism will play very badly indeed. Despite their success in the No to AV campaign, the last people I would involve are the Taxpayers’ Alliance!

There must be a Plan B, though. This involves a discussion about the constitutional future of England – a pluralistic country that should not be subjected to never-ending Conservative majoritarian rule. Radical constitutional reform would be very much back on the table in the event of independence. If Scotland even opts for fiscal independence, clamours for an English Parliament may become impossible to resist. Labour must begin to think the unthinkable and start to ask how such a Parliament could work. It would certainly have to be elected by proportional representation (and the Mixed Member System seems to be the only runner in this regard; people like crosses not numbers to be brutally honest). 

Either way, the discussion must start now. Devolution was always an unstable compromise in the best, ironically, English tradition. The Scottish will have their debate. England must also have its debate. Why union? Why must it endure? How can we make it work for all of its constituent nations in the best way possible? Just as the AV campaign seemed to have taken major constitutional reform off the table, Alex Salmond has put it back on again with his stunning election victory. The UK constitution is far from inert; it’s high reactive indeed and we won’t be able to shy away from that. One spark and the whole lot could blow… better to take precautions.