A Venetian ambassador to London described the young Henry VIII as “the handsomest potentate I ever set eyes on ... his complexion very fair and bright, with auburn hair combed straight in the French fashion; and a round face so beautiful that it would become a pretty woman, his throat being rather long and thin...” The Young Henry VIII, painted by an anonymous English court artist in around 1513, is exhibit 43 of 275 in the British Library’s sprawling new exhibition, “Henry VIII: Man and Monarch”. The picture is by no means a masterpiece, but it conveys something of the surprising delicacy noted by the young king’s contemporaries. He wears his hair in an effeminate mop, loose strands pinned into place by great bejewelled clasps. Between thumb and forefinger of his right hand he holds a rose, symbol of the Tudor dynasty. His skin is porcelain pale and he has a faraway look in his eyes.

Guest-curated by David Starkey, the British Library’s exhibition marks the five hundredth anniversary of Henry VIII’s accession to the throne. Starkey is known as a popular historian, but despite the window-dressing of a ready-to-mount virtual jousting machine – complete with video-enabled visor – this is a heavyweight historical show. The emphasis, throughout, is on primary textual sources, so that for long stretches most of the exhibits take the form of manuscripts, letters and legal documents. In fact, one of the revelations of the display is the inverse relationship between Henry’s growing autocratic ruthlessness and his ever-increasing obsession with legal technicalities. Not content with breaking his Coronation Oath, Henry amended the actual document itself after the fact, as if to change the laws that he himself had violated. Such scribblings might look naive and clumsy, but it was through those crossings-out that England itself would be utterly transformed (elsewhere, there is a hit-list of monasteries compiled by the King’s “assessors”, in which every word stands for a great religious foundation, soon to be extinguished).

Henry’s squat, heavy handwriting also appears in the margins of his personal copy of The Bishops’ Book of 1537, altering the text of two of the Ten Commandments to suit his own preferences and behaviour. On this occasion, a rarity, Henry’s amendments to law went unregarded. Archbishop Cranmer was given the unenviable job of pointing out that even the King of England did not have the right to redraft the tablets handed down by God to the Prophet Moses.

An entire section of the show is given over to the vast library of Anglo-Saxon laws, Roman law and conciliary degrees taken from monasteries in the 1530s and deposited in the Royal Library to feed the so-called Collectanea satis copiosa (“the sufficiently full collections”) – an armoury of legal sources gathered together in support of the idea that Henry should exercise supreme jurisdiction, both secular and spiritual, within “the empire that is England”. There are no works of art, in this part of the exhibition, yet to look through these seemingly dry and dessicated texts is to leaf through the long sentence of execution passed, by Henry, on almost the entirety of  English medieval painting and sculpture. The great goal was divorce from Katherine of Aragon, which the Pope determinedly withheld. But from the Collectanea stemmed the break with Catholic Rome, the death of Thomas More and the rise of Thomas Cromwell. From those events, in turn, stemmed the Dissolution of the Monasteries, the Reformation of the church, and the wholesale destruction of thousands upon thousands of images of Christ, the Virgin Mary and the saints in heaven – all decried as “false idols” by the revolutionary Protestant clerics of Henry’s new “Church of England”. As a token gesture in the direction of that vast, black hole of destruction, a few shattered stone heads from Glastonbury have been included elsewhere.

What other images there are in the display are prompts for melancholy reflection. An impressive gallery of ghosts has been assembled, mostly of those whom Henry first favoured, then executed. There is kind, smiling, Thomas More, drawn with patient affection by the inimitably brilliant Hans Holbein. There is stern Thomas Cromwell, sober in black, the epitome of Tudor realpolitik (until his death, that is, when he wrote tearfully to Henry pleading for “mercie, mercie, mercie”). There is the sad-faced poet, Henry Howard, of Surrey; and, of course, the famous harem of doomed queens.

The exhibition appears to have been conceived in admiration of its subject, but the overall effect is hardly eulogistic. The picture is painted of an expedient tyrant with megalomaniac delusions. Henry was a forceful king, for sure. He transformed the spiritual, cultural and political landscape of the nation, but he did so with such strange and whimsical fitfulness that most of the revolutions he initiated must be counted the accidental consequences of raggedly impulsive acts.

The king’s own image is threaded through the show, its metamorphoses tracing the story of an ineluctable decline. The young man with the long neck and the round, feminine face soon turns into the secular God, worshipped by his terrified people, immortalised in the famous group portrait of Henry VIII and the Barber Surgeons. The very last picture of him, Quentin Massys’s The King in His Fifties, is most damning of all. Enormously overweight, his face a jowly mass of shivering royal flesh, Henry in his last days appears to have metamorphosed into a kind of toad. He peers out of slitty eyes as he waits for death to take him.