Are there theories as to why so much of Enlightenment political thought (for examples Hobbes, Locke, Smith) emerged out of England in the 17th and 18th centuries? Was there something distinctive about how openly these subjects were permitted to be discussed in the public square?

The way that this question is framed reveals something interesting about the popular understanding not only of British history, but about ideas surrounding political theory more generally. In many circles, even academic circles, "British" has come to be synonymous with "English" since at least the nineteenth century, when the lesser partners of the British Union were relegated to unimportance by the focus on the grand narrative of English progress. These attitudes have continued on into the twenty-first century; Scottish history (let alone Welsh history) has been deeply marginalized by the academic establishment to the point where many academic presses won't publish titles in Scottish history because they view them as too "niche". Thus, the books that are available for the reading public to consume are invariably focused on English history that has just been marketed as "British" to sound more inclusive. What this means is that curious history lovers will ask questions about the "English Enlightenment", when in reality, England contributed very little to Enlightenment debates. Instead, it was primarily French and Scottish writers who shaped the discourses that would later incite political revolution across Europe and North America.

This leads us to the next misconception that is often tied to political theory of the early modern period. While the eighteenth-century Enlightenment certainly did have an impact upon the eruption of the American War for Independence and the French Revolution, early modern political theory was not inherently oppositional or combative to the reigning establishment.

You mention Hobbes and Locke here, both of whom were English, and Adam Smith, who was Scottish, and who is remembered more for his economic theory than his straight political theory; however, in order to understand their political viewpoints, it is necessary to focus our attention first to the social changes occurring across Europe in the sixteenth century.

By the dawning of the sixteenth century, Humanist thought had already taken root in the various Italian city-states and had started to gain traction across Germany, the low countries, and the British Isles as well. As a movement deeply concerned with what it meant to be human and to live morally, Humanism took as its primary references the works of Roman statesmen and philosophers, particularly the works of Cicero. That said, this interest in classical thought flourished and Humanist writers drew extensively from medieval traditions of Aristotelianism and new traditions of "neo-Roman" theorizing, and, further Greek thought as represented by the works of Plato. The greatest political debate to emerge from this was whether a moral man had a civic duty to live an active political life, providing counsel to the monarch, or, whether a moral man should retire to stoic isolation and refuse to become embroiled in political maneuverings, which were viewed as having a corrupting influence. You can see these debates in the writings of Erasmus and in the way that Thomas More effectively critiques Platonic attitudes in Utopia. In the end, the idea that men had a civic duty to engage in the political community surrounding their monarch tended to win out over the argument that it was more moral to retire into seclusion and avoid any temptations of political engagement.

Compounding the debates introduced by Humanist thought were the Protestant and Catholic Reformations of the sixteenth century. What is moral when the monarch might be of a different confession than the rest of their kingdom? That is, in the cases of Mary I of England and Mary, Queen of Scots, both women were Catholic while their kingdoms had embraced the Protestant faith. This kind of situation encouraged political thinkers in both England and Scotland to begin to dabble with resistance theory in order to justify very limited rebellion against the authority of a monarch who either persecuted their own subjects as heretics (Mary I) or refused to officially recognize the Reformed Kirk (Mary, Queen of Scots), which left the Kirk in some kind of liminal space in which it lacked official status. It's important here to make clear that none of the early modern political theories that developed in Britain advocated republicanism or pure democracy. At most, they called for combined governments in which the monarch was forced to accept the counsel of advisors (effectively, Parliament), and to rule in concert with these advisors. It wasn't really until the disastrous reign of Charles I that political theory began to grow more radical, and that was entirely in response to the perceived tyranny and misrule of the sitting monarch.

So, to answer your question in more explicit terms: the rise of political discourse in the early modern period can be attributed to the rise of Humanism in conjunction with the changes wrought by the various sixteenth-century Reformations. It is important to note as well that in the early modern period, the only people engaging in these debates were, at minimum, members of an educated middle class and other political elites (usually men drawn from the landed gentry or nobility). These debates circulated primarily in print or manuscript form and were not welcoming of the input of women or the "lower" social orders. So, the idea of these theories being discussed in the public square is an anachronism at best. That said, the first English coffee houses became breeding grounds for political discussion during the seventeenth century. After the Restoration, coffee houses were in fact closed because it was believed that they bred seditious rhetoric. Effectively, political debate and theory was intended to be exercised only by a select few in response to the unique circumstances that arose from social change and the personal characteristics of particular monarchs.

As I am a scholar of early modern Britain (Scotland, but also England), I have not addressed the rise of political theorizing and debate in France, which was, arguably, the primary seat of the Enlightenment in the eighteenth century, because I don't know enough about the unique circumstances in France to tackle the subject in a meaningful way. I hope, though, that a member of this community, flair or otherwise, will be able to weigh in and add insight on France to the discussion.

For further reading, I'd recommend checking out the following volumes: