Wings over Scotland recently published an interesting article which contained the following illuminating passage:
So why would the UK deliberately undermine the long-held view that the UK is a political union of different countries? The answer may be seen in a passage from the report stating that "Since the rUK (remainder of the UK) would be the same state as the UK, its EU membership would continue", and that after independence, representatives of the UK Government would enter negotiations on the terms of independence "as representatives of the continuing state of the UK".
From these two snippets it appears that the repositioning of the Act of Union as merely an enlargement of England is an attempt to retain sole-successor status in the same manner as Russia after the collapse of the Soviet Union. The Westminster government is so desperate to keep hold of the permanent Security Council seat that they’re willing to undermine the constitutional arrangements of the UK in order to ensure they keep it in the event of a Yes vote.
I'm not an expert on UN membership rules, but I would have thought there was a decent chance the rUK will retain the UK's permanent seat on the Security Council. However, even a modest risk of losing that seat is probably enough to give the politicians and mandarins in the FCO and the rest of Westminster sleepless nights. Sacrificing the happiness and wellbeing of the Scots is a very small price to pay for maintaining a place amongst the great powers of the world.
Besides, the unionist politicians in Westminster are not the only ones who are worried. David Leask quotes Phillips O’Brien of Glasgow University for the following: "France’s place in the world would come under real pressure if Scotland were to leave the United Kingdom[.] In the first place, it could lead to reform of the UN Security Council and the concurrent loss or reduction of French influence in the UN."
Personally I'm pretty relaxed about a reform of the Security Council, but I can understand that for a small group of politicians clinging to the remnants of the empire, it can seem like the end of the world as they know it, which explains why they attack Scottish independence so vociferously.