This year marks 100th anniversary of the brave attempt on the South Pole by the Terra Nova Expedition, led by Robert Falcon Scott. Of the five men to make it to the Pole, three were English, one Scottish and one Welsh. The awesome feats of their endurance and sacrifice were not just a result of true comradeship. They were a reflection of the great, unifying pride in the achievements of Britain held throughout these islands at the beginning of the last century. It is sad to reflect that, in just two years time, the seal might be set on that nation's end.

For the Acts of Union did not merely create a political entity. They affirmed the existence of an over-arching nation: Great Britain. 'Affirmed' because the idea of the nation of Britain was one whose origins are lost in the dawn of recorded history. This island was first recorded as 'Pritain' before the Roman invasion. An earlier name for the island - Albany - is itself the root of the 'Alba' used by the first Gaelic Kings as the name of a unified Caledonia in the 10th century, a recognition that the three ethnicities that first made up the kingdom - Gaelic (Scots), Pictish and (Strathclyde) British - were united more by the land than by immediate common bonds of kinship.

The last ethnic group to join the late-medieval Kingdom of Scotland was actually English. Edinburgh was itself an English town (more precisely, an Angle town), a part of the Kingdom of Northumbria from the seventh to the 10th centuries, when the eastern Borders were settled by this Anglo-Saxon tribe. It was this form of Old-English that developed into Scots before Modern English largely replaced it from the 18th century.

This linguistic unity has been the corner stone of England's and Scotland's gradual but resoundingly successful pooling of their identities into a greater whole. Through a common language were ties between the nations able to grow: the common adoption of Protestantism, the adoption of one English language bible (first the Geneva Bible supported by John Knox before the Authorised Version was introduced across Britain) and the shared experience of the civil wars all brought together two kingdoms united in a personal union from 1603. The great trading and pooling of ideas, assisted by the printed word, that marked first Renaissance and then Enlightenment accelerated a process that led to what really was a shared British culture by the 18th century.

And it is this shared understanding, this common linguistic, cultural and religious heritage, that was the foundation of the common enterprise - the commonwealth - of Britain. Though Empire has had its part to play - the disproportionate role of Scots and Irish in the military and trading opportunities of Empire was particularly notable - the kingdom was able to grow in its unity because of a mutual loyalty able to rise above admittedly ancient divisions. This is not to deny the terrible legacy of Glencoe, Culloden and the Highland clearances; nor that those acts hindered the integration of Gaelic Scotland into a commonwealth of Britain. But history has, mercifully, been more prone to forgive and forget in Scotland than it has in Ireland; and the dynamic celebration of Highland culture - indeed, its adoption by the rest of Scotland as a common national heritage - has complemented, rather than competed with, a fundamental sense of loyalty to the notion of Britain as a unifying, political expression.

Our political parties are as much Scottish as English. Labour's founding father, Keir Hardie, and first Premier, McDonald, were Scots, as was the greatest Liberal Prime Minister, Gladstone, and Conservative Premiers such as Balfour and Douglas-Home. The thinkers Adam Smith and David Hume, whilst figures of the Scottish Enlightenment, were some of the most important contributors to the developing broad liberalism that was to define the English speaking world. (Their contemporary, Burke, was himself an Irishman.)

It is hard to imagine the British Army - an institution younger than the personal union - shorn of the Black Watch; the Royal Navy denied Rosythe and Scapa Flow; or the RAF unable to operate from our northernmost shores. It is for good reason that the Prime Minister today outlines the advantages we all derive from the security we build together. Those narrow English nationalists might want to consider how effective a nation we would be without the immense Scottish contribution to our national security. (As an aside, it is difficult to see many Scottish soldiers - the hardiest of all - being particularly enchanted with the non-NATO peacekeeping role Mr Salmond has in store for them.)

Yet, as David Cameron argues, this is so much more than an actuarial calculation of relative advantage and disadvantage. It is a question that speaks to our hearts: a question of home. However fierce our rivalry, how would we really feel to be leaving our country at Gretna Green or Carlisle? How many of us would be faced with an agonising conflict of loyalties when forced to make a choice that we thought would never be necessary.

As an Englishman and an Irishman - but a Briton first - I look upon this debate with the despondency of one who wishes the United Kingdom could have remained wider still. The sadness of division in Ireland, both within and without, is nothing to envy. We, the British people, are greater than the sum of our parts. Let us fight, wherever we are, for our shared national home.

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