I have the following on that thus far:

As with so many other examples of British tensions, the difficulty of uniting England and Scotland stems in large part from issues of the monarchy. A century before the Treaty of Union, James IV of Scotland and I of England came to rule over the two countries (and Ireland): "The experience of the 'regal union' or 'union of the crowns', stemming from 1603 when James VI of Scotland had succeeded his mother's childless cousin Elizabeth, Queen of England, thus beginning an often unhappy arrangement by which two historically antagonistic kingdoms shared the same monarch, lay at the root of the strained relationship." - David Allen, ‘Nation,’ in Scotland in the Eighteenth Century (London: Longman, 2002), p. 3 As such, the seed of a British union had been planted in the beginning of the seventeenth century, but the English parliament rejected the proposal in 1607. Scottish estates favoured a federative union rather than full incorporation into the English parliament. A number of subsequent attempts to unite the countries either commercially or politically shows a persistent if unsuccessful effort to reach an accord. (Macinnes) England and Scotland remained separate sovereign states, however, and this regal union was plagued by troubles and doubts from either side. A political and military crisis of 1630s and 1640s culminated in the 'Bishops' Wars' in 1642 (Allen, p. 4) under Charles I. James' grandson Charles II also failed to convince English and Scottish statesmen that "royal proposals for economic union (1668) and parliamentary union (1670) were in their mutual interest." (Allen, 4)