We have many derelict mine works in Cornwall, because it was the most prolific tin, copper and china clay pits in the UK, from the time of the Industrial Revolution then tailing off around the 1980s. There are a few of the Engine Houses still intact - they have some small museums and/or coffee shop added on.

With the Megalithic sites, usually they are protected by laws, so they can be seen/touched, but not 'disney-fied' eg plastic keyrings etc. That is left to the holiday resort areas. There are plenty of stone circles from the Megalithic times in Cornwall, left over from a more Celtic/pagan time :-)