There's really not a lot of places that you can point to and say 'this is Arthur's Britain', mainly because no one can agree on whether Arthur even existed, and if he did whether he was a northern figure or a western! These are the darkest of dark ages! However, I believe, on what little evidence I found, that he operated out of the west country, so I'd advise you to visit Caerleon in south Wales, because there you'll find a Roman amphitheatre that he must have known. There's a strong legend that 'Camelot' was at South Cadbury in Wiltshire, and that's worth a visit anyway. It's an Iron-Age hillfort, but recent archaeological work has shown that it was re-fortified in the Arthurian period, so it has a strong claim to being a fortress that the Britons would have used in their fight against the Saxons. As for the rest, drift around Wiltshire, Dorset, Somerset! Go to Avebury before Stonehenge, they have little to do with Arthur, but they were there long before he was, so he might have seen them. It's really a question of soaking up atmosphere. As for the site of the crucial event in the period - the battle of Mount Badon - no historian denies it took place, but no one knows where. I venture Little Solsbury Hill just to the east of Bath, and that's worth a visit too.