The role of the knight in Britain's society has changed throughout the centuries. Nowadays, Britain is one of the only countries to still uphold the ancient tradition of awarding knighthoods to exemplary individuals who distinguish themselves in the name of their country. However, in the early to middle Medieval period, knights were much more common! In fact, they were a crucial part of the Feudal system: England's system of governance since the invasion of William the Conqueror unto the late Middle Ages.
The Feudal system worked thusly: the King owned the land, and he would loan it out to the Barons and the Earls, who would loan it out to the Knights, who would loan it out to the Peasants that worked the land. As you can see, Knights were landowners though they did also have martial responsibilities and were required to serve as soldiers in the country's wars, as and when they arose. (edited)
By the time of the 13th century, there was a fairly strict process to becoming a knight that became uniform throughout the Medieval world, pioneered by the practices and traditions of England's noble class. Young boys from well-to-do families would be sent off at age 11 or so to the houses of other nobles, where they would become pages, effectively boy servants tasked with carrying out menial errands and such while being taught in the skills of writing, reading and swordplay.
Around the age of 14 they would eventually be granted the title of Squire and tasked with shadowing a superior Knight, looking after his armour and cooking meals for him etc. Squires also saw real combat fairly often, and this was seen as the true test of their valour and if they had earned their knighthood or not - many squires died a gruesome death on the battlefield. For those that survived however, after years of service, during his late teens or early twenties a successful squire would finally be given his knighthood. This could theoretically be granted by any knight, but the ceremony was often conducted by the local reigning lord.
Through the years, the Medieval kings of England were met with great reluctance from their knightly noble subordinates when it came to service in the military - understandably so! As such, during the 14th century, numerous royal edicts were created that ordered the knighting of every man who owned land of a value above £20 (a wealthy sum in those times), and then twenty years later the same order was made for lands worth over £40. This shows how knighthood evolved through time out of necessity to revolve around ones personal wealth and financial status, as opposed to their lineage!
As time went on, the cities of Medieval England expanded greatly and so too did the population, with London's own population growing by 500% over the course of just a couple of centuries. As such, the idea of a standing English army that would remain active through both peacetime and war became much more plausible and realistic. This meant that there was less and less of a need for Knights to be called up to serve in the military, and eventually the status of Knighthood was more of an economic than martial role. However, the upper class did try their hardest to prevent this - through parading chivalrous ideals, holding tournaments and other displays of valour, and so on.
While chivalrous ideals were far much more than a fairytale and very much considered to be apart of a Knight's everyday responsibilities to both himself and his country, the reality is that these values were upheld far less than one might think. In fact, the closest these ideals ever came to realisation was during the Crusades - military campaigns to the Holy Land in which England played a crucial role, specifically in the Third Crusade under the leadership of King Richard the Lionheart. However, during peacetime, knights were known to take advantage of their favourable situation to live as greedily and selfishly as they wished, and not at all for their generosity, honour or martial ability. As with many popularised historical concepts that have found their way into pop culture, the modern public's general idea of the old English knight as opposed to the reality of one, is often very very different.