There is copper mine open to the public on the Great Orme in
North Wales, we went there a couple of years ago whilst staying nearby, it's very interesting and you are allowed to go underground without a guide, passages are quite low in parts so being rather tall and wearing a hard hat I ended up with backache
Info from the site:-
Uncovered in 1987 during a scheme to landscape an
area of the Great Orme, the copper mines discovered represent one of the most astounding archaeological discoveries of recent times. Dating back 4,000 years to the
Bronze Age they change our views about the ancient people of
Britain and their civilized and structured society 2,000 years before the
Roman invasion.
Over the past 28 years mining engineers, cavers and archaeologists have been slowly uncovering more tunnels and large areas of the surface landscape to reveal what is now thought to be the largest prehistoric mine, so far discovered in the world.