To give a very brief answer, Scotland and England's direct union in 1707 came as a result in large part of Scotland's catastrophic failure to establish a colony in the Caribbean, which bankrupted many of its nobles and effectively forced it to default upon its debt. At this time, Scotland and England had been in what was effectively a personal union for upwards of a hundred years, discounting the period of the civil war and commonwealth; the actual tension between the two countries was far more a matter of nationalist prejudice than it was an event of open hostility between the two nations. But the popular opinion of the union varied significantly; for England, it was an opportunity to ensure that Scotland would choose the same monarch as it did in the aftermath of the changes to the succession introduced in England to ensure a protestant heir inherent to the act of union (as by uniting Scotland the two parliaments would be united into a single one, ensuring that the same monarch would rule over both Scotland and England; but for Scotland, opinions varied far more. While aristocratic Scotsmen had much to gain financially for the union - including reimbursement for the disaster of their failed colony, Caledonia, ordinary Scotsmen were far more opposed to the union; Burns' denouncement that 'We are bought and sold for English Gold' is instructive here.

Further evidence of popular resentment can be seen in the fact that the Jacobites always sought to invade England through Scotland; although their support there was by no means unanimous, the fact that on two separate occasions they effectively subverted English rule in Scotland further shows a divide of opinions about the Union.

I don't know offhand how popular it was at the outset, but I can recommend two books that deal with the aftermath of 1707. One is Linda Colley's Britons, in which she argues that Scottish, Welsh, and English people came to think of themselves as "Britons," as well as Scottish, Welsh, and English, in the century and more after 1707. This British identity was formed in the long series of wars between Britain and France from 1689 and 1815, and Britons constructed their identity as "free," Protestant, prosperous, and uniquely blessed people in opposition to an imagined tyrannical, Popish France.

John Brewer's book The Sinews of Power tracks the ways that the British state was able to more or less win that series of wars because it developed the machinery necessary to finance them. Between about 1700 and 1800, the state's debt essentially doubled with every war. By the 1720s, they gave up worrying about trying to ever really pay off the debt, and concentrated on simply paying the interest. In this way, the banking establishment of the City of London and the British state collaborated to develop what Brewer calls a "fiscal-military state," a bureaucratic and governmental apparatus designed to collect taxes, borrow money, and fight wars. They developed long-term loans which were tied to specific excise taxes, so that loaning money to the government became a long-term investment, guaranteed by the state's ability to collect taxes. This gave the state access to cash, which they used to finance constant wars.

The interesting thing about this fiscal-military state, however, is that it actually helped to tie the United Kingdom together. It provided an avenue for investment in the state, but that very investment gave many people of substance in both kingdoms very clear interests in maintaining the new United Kingdom. Allowing England and Scotland to fracture into war again--as might have happened if, say, the 1745 Jacobite uprising were successful--would have jeopardized their investments. So, while the long series of wars against France may have encouraged Britons to think of their cultural similarities, that same series of wars also provided means for the integration of institutions, creating a substantial and powerful class of people with deep interests in keeping the Union successful.

So, the takeaway from these two books is that the popularity and institutional stability of the new United Kingdom developed over the eighteenth century. Perhaps not surprisingly, the union of England, Wales, and Scotland was quite stable by the early nineteenth century--perhaps more at that point than even before or since.

Really, we need our resident Jacobite specialist, /u/lngwstksgk, to help us out here.

From Herman's The Scottish Enlightenment. Daniel Defoe reported that the talk on Edinburgh's streets was of slavery to the English and taking away the nation.

There were riots in Edinburgh, which was placed under martial law. Rioting then spread to Glasgow and Dumfries. When the members of parliament met to sign the treaty, the mob forced them to flee. They then tried to meet to sign the treaty in a tavern and a summer house. each time they were spotted and had to run for their lives. They ended up signing it in a basement. Then everyone left for London that night.

As a follow up I have always wondered why when Wales and England joined they merged the legal systems but Scotland got to keep theirs. Could anyone explain why Scotland was different?

Thanks everyone for excellent answers. I have some reading to do.