In England, the gradual weakening of the central government (starting with the Magna Carta), and the gradual rights given to nobility slowly pushed England to a democratic form of government. Yet in other countries with a weak central government (Poland, Holy Roman Empire), no democracy ever developed, and the nobility just upgraded their status to that of Kings.
What was unique in England that pushed democracy?
I wouldn't characterize post-Magna Carta England as having a weak central government. Compared to the Holy Roman Empire it had a very efficient central government, in which the parliament played an important role alongeside the king.
The early English Parliament already had a House of Commons. Hence not only the nobility was given rights but the common people too. Of course, the elections to the House of Commons were far from democractic in the modern sense, with voting rights limited by land or property ownership requirements.
Besides the Magna Carta in 1215, a number of key events in the development of English/British democracy must be noted.
The Second Baron's War in 1264 led to the creation of Simon de Monfort's "model parliament". Although the rebellion eventually failed, the parliament set a precedent for the future.
The English Civil War in 1642 led to the temporary establishment of the Commonwealth of England, a republic. Besides early republican ideals, an important factor in the revolution was religious (Puritan). The Restoration in 1660 returned the monarchy but the power balance permanently shifted towards the parliament.
The Glorious Revolution of 1688 resulted in the establishment of William III and Mary II in place of James II. The further (significant) weakening of the monarchy was codified in the Bill of Rights of 1689. The Glorious Revolution marked a clear victory of the social contract theory of government over the divine right of kings. The formation of proto-democratic ideology accelerated due to the efforts of thinkers like John Locke.
The Reform Act 1832 widened electoral suffrage significantly and eliminated corrupted practices such as "rotten boroughs" and "pocket boroughs" which allowed a small group of powerful men to control a large number of seats in the Parliament. This was already after the American and French revolutions, and republican ideals were becoming widespread in the western world. England has had a large number of its own liberal thinkers such as the radical Richard Price and the more conservative Edmund Burke. Further electoral reform was stalled for a while although promoted by movements such as Chartism (founded by the People's Charter of 1838). Eventually reform continued with the Reform Acts of 1867 and 1884.
Representation of the People Act 1918 granted almost universal male suffrage and limited female suffrage.
Representation of the People Act 1928 granted females and males equal voting rights.
In addition to what you list, the organizational structure, bookkeeping of the monarchs, and relative literacy levels (albeit not high levels absolutely) helped enable a democratic system to emerge.
In addition to the points already made, I would say that religion might have played a major role in how England turned out. The English Civil War and the Glorious Revolution, both of which severely damaged the power of the monarch (the former completely destroying it for a while) were partly because of religion.
Structures like the English Parliament were already in place to take advantage of the weakening of the power of the monarch or the nobility for that matter.
A second reason I stress the above two events is that even as late as the mid-16th century (end of the reign of Henry the VIII), the monarch had an extreme amount of power and democracy really started emerging during the mid-17th century.
There was a medieval saying, "Stadluft macht Frei." (City air makes one free.)
One important aspect of England (and Greece and Rome before it) was the relative urbanization of its time. The most "progressive" and democratic elements of society tend to concentrate in cities, whereas the most conservative and pro monarchic influences are generally found in the countryside.
A major test of England's progress toward democracy was the English Civil War of the 1640s, which at its core, pitted the new urban elite against the agrarian nobility. (The former won.) Cities like London and York (to a lesser extent, Liverpool and Bristol) housed manufacturing interests that tended to support democracy, or at least provide protection from the feudal lords. In Poland and the Holy Roman Empire, there were fewer cities representing a smaller proportion of the population keeping the nobles in check.
It wasn't just "weakening of central government", but more specifically, weakening of the King/Queen, giving Parliament more power. The Magna Carta was part of that.
Another notable event was the Glorious Revolution in 1688 when Parliament kicked the (Catholic) King James II out and replaced him with the Dutch. This is another example of Parliament (i.e. the nobility) limiting the powers of the King.
Over the years, Parliament got more democratic as they allowed more people to vote, cf. Reform Act
Even earlier than the Glorious Revolution, I recall reading the Hundred Years' War between the royal dynasties of England and France from 1337 to 1453 also played an important role in building the foundations of democracy in England.
The English kings were forced to finance the war effort (which was not universally popular) with the aid of Parliament and thus conceded authority to that early democratic institution. England's ultimate defeat in the Hundred Years' War was partially responsible for the War of the Roses which split the English nobility into opposing factions and further weakened the Crown.
In contrast, the Hundred Years' War allowed the French monarchy to increase its central authority and taxation powers for the defense of the home soil. The French kings were less dependent on national legislatures for raising money than England. In fact, Charles VII established a standing army in 1445, which England did not have for another two hundred years.
