1 Prof. Jones: Maybe we've discovered Arthur's Round Table! Wouldn't that be something?
2 Monty: You're conflating Arthur's alleged birthplace of Tintagel with his court at Camelot. The Round Table wouldn't be here, even if it existed.
3 Prof. Jones: We don't know where Camelot was. It could have been the same place! What makes you so sure this isn't the Round Table?
4 Monty: Well, it's square. And it's a chair.

King Arthur famously had a Round Table at his court in the castle of Camelot, this being the table at which he and his knights would sit to discuss the affairs of state and the kingdom. The idea of the table being round was that it had no head of the table, traditionally where the most powerful person present would sit. This symbolically implied that all people seated around the table were equals, which was a sign of Arthur's wisdom and magnanimity in understanding that his knights were as important to maintaining order in the kingdom as he was.

Camelot, being (we think) a mythical place, has no definitive location within the British Isles. One popular theory places it at Winchester, some 100 km south-west of London^[1]. One thing Winchester has going for it is that it has an ancient castle, and within the only remaining part of it - the more recently added Great Hall - can be found an old round table, decorated with a painting of King Arthur and the names of his knights around the edge:

Although it looks impressive, this table is actually an artefact of the 13th century, some 700 or so years after Arthur's mythical reign. The wood dates from that period, but the painting was done in the 16th century on the orders of King Henry VIII, as could be guessed by the prominent Tudor rose in the middle, and the fact that "King Arthur" bears a striking resemblance to King Henry VIII himself. Henry was presumably enamoured with the story of King Arthur, and fancied himself in that mould.

[1] And, coincidentally, the place where I spent my very first night ever in England. My wife and I flew into Heathrow Airport, picked up a hire care, and immediately drove away from London, our intention being to take a tour of the countryside first and finish our holiday in London at the end. A few hours later we ended up in Winchester, and decided that was enough driving for jetlagged people, so stopped to spend the night. It turned out to be a wonderful place to stay and explore.