Tomorrow, in St Andrews House in Edinburgh, before the world's press and media, Prime Minister David Cameron and First Minister Alex Salmond are expected to sign a historic agreement to hold a legally-binding referendum on independence for Scotland to take place no later than October 2014.

We welcome unreservedly this agreement, which puts an end to months of speculation about the details of how Britain's constitutional future will be decided. We further welcome the decision to put a single straightforward question to the Scottish people asking whether or not they wish to become an independent state. We further applaud the apparent agreement to allow 16 and 17-year-olds to participate in this historic decision, which will have a profound impact on their lives. If adults are old enough to join the army, marry, have children and own their own home, we feel they are mature enough to decide in what country they wish to live.

The supporters of federalism, devolution max, devolution plus or any of the other halfway houses may feel they have been disenfranchised by the agreement to put a single in-or-out question. After all, opinion polls consistently suggest a majority of Scots wish neither the status quo nor outright independence, but favour a devolved parliament with greater economic powers. There is a strong argument that a referendum should include the option for expressing that view. However, the stronger argument is that clarity is everything in constitutional affairs. No-one would benefit from a referendum result that would allow both sides to claim victory and lead to interminable wrangling over the interpretation of the vote.

The widening of devolution could be debated if – and it remains a large if – the referendum delivered a rejection of independence. The Scottish people must first decide, once and for all, whether or not they wish to remain in the United Kingdom as presently constituted or wish to become an independent state similar to Norway, Denmark or any of the other small nations of Europe.

The most recent opinion polls may convince the No campaign that it will win a convincing victory and that Scotland will remain in the United Kingdom. We are not so sure. There are strong arguments on both sides, and the mood of the Scottish people in 2014 may be very different to the mood today, which has been influenced by the Jubilee celebrations and particularly the London Olympic Games. It has been apparent for some years that Scottish political culture is growing apart from that of England in areas such as education policy and health provision. The Scottish people remain broadly in favour of universalism in areas such as higher education, prescription charges and free personal care. There is frustration in Scotland at the concentration of economic and political life in London and the south-east of England, which has for decades denuded Scotland of capital and talent. This country has a legitimate grievance that revenues from North Sea Oil have not benefited Scotland as they have Norway.

However, it is for the Yes campaign to explain just how independent Scotland would really be while retaining the pound and allowing the Bank of England to continue to set interest rates. Alex Salmond insists that an independent Scotland would remain in a "social union" with England, and that Scots could still regard themselves as British. But if this is the case, why is it necessary for Scotland to leave the United Kingdom and for Scottish MPs to withdraw from Westminster? Could this new UK not be forged by incremental devolution? After all, Scotland remained a nation after the 1707 the Act of Union, and lost its parliament until 1999. Could the United Kingdom not be reformed from within?

It is now time to address these issues seriously. This is a momentous decision. In two years' time the United Kingdom could cease to exist in its present form. It is to David Cameron's credit that he has not tried to prevent Scotland deciding on its own future in a legally binding referendum. One only needs to look at the turmoil in Catalonia, where a referendum has been rejected by Madrid, to see the dangers of suppressing democratic national sentiment. Fortunately, we do things differently here.