Monks at Glastonbury Abbey claimed to have found the remains of Arthur, after the old wooden abbey burned down, and the monks were destitute.
Glastonbury Tor is widely believed to be the original Isle of Avalon. The area surrounding Glastonbury was marsh land, until it was drained in the 13th Century to convert it to arable farm land.
So at the time concerned, Glastonbury would have looked like an island, and its striking Tor has been of religious significance since atleast the Druids, maybe even as far back as the days before Stonehenge was build.
So penniless monks, with no monastery, living on the site believed to be Avalon, would find it very fortuitous to uncover the remains of the most legendary of British kings. Especially during the reign of Edward I whose fascination with the Arthurian legends, was only exceeded among English kings, by Edward III who based the Order of the Garter on the Round Table, and even began building a Round Table building within the grounds of Windsor Castle.
Edward I actually attended the re-burial ceremony of the alleged remains of Arthur, a century after they were first discovered. Needless to say, this discovery turned the fortunes of Glastonbury Abbey around after the fire.
Relics were big business in the middle ages, the vast majority of relics were fakes. John Calvin once commented that if all the relics were assembled "it would be made manifest that each Apostle had more than four bodies, and each Saint had two or three"
Obviously King Arthur was not a Biblical character, but in the Middle Ages, he was as big. Even to this day, people visit Glastonbury Abbey, and put flowers in his alleged grave.
So the grave at Glastonbury Abbey may be the final resting place of King Arthur, or it could be a pious fraud by homeless monks.
Even with this grave, his existence remains a mystery. Maybe its better left that way. For if he was a real historical person, and we find out who it was, then he would be unlikely to have lived up to the legends that he inspired.
The king who came closest to living up to the Arthurian Legend, was Ælfred the Great. The only monarch in the British Isles to ever be called "the Great", and it was a well earned epithet at that. But he never commanded the global fascination, nor had as many stories and books written about him as Arthur, despite him laying the foundations for the unification of England.
The reason for that is simple, their is no mystery behind King Ælfred, we know he existed.
Arthur can't ever be proved to have not existed, because you can't prove a negative, but in such a case, the legend would be in danger of becoming just another fable or fairy tale.