There is a surprising amount of rock art in Britain. Some of the more famous are cup and ring marks in Northumberland, the Cheddar mammoth, and the carvings at Creswell Crags, but there are thousands of sites all over the country.
Various theories have been advanced for what rock art is for. Could cup and ring marks be maps of sacred springs? Or maps of constellations? Or records of shamanic journeys or trace states? Or visual representations of sound? Or just art for art's sake?
Animals are sometimes seen to emerge from cracks in the rock, as if emerging from the spirit world behind the rock wall (it is known from ethnographic parallels that the spirit world was believed to be in the rock).
It's amazing to think that these marks have been there for thousands of years in the wind and the rain, and that they were carved without the use of metal tools.
Sadly some rock art has recently been vandalised. I very much hope that the vandalism was not done by Pagans. There was a story recently of someone (not a Pagan) chipping bits off a stone circle for use in homoeopathic medicine, but the vast majority of Pagans would not do this. Because of the eclectic nature of Paganism and the widely distributed networks of involvement, it's difficult to reach everyone involved and tell them not to do this kind of thing (not everyone who identifies as Pagan reads the same magazines or attends conferences where the message "take only photographs" is promoted). However most recent vandalism of stone circles and the like was not perpetrated by Pagans. I know people sometimes leave tealights and flowers and even chalk-marks (there is an ongoing effort to educate people not to do this) but actually carving on the monuments is rare. People should never ever chip bits off megaliths, or add any extra carvings or marks. It's best not to leave offerings either, even biodegradable ones (if you do want to pour a libation to the spirit of place, do it well away from any megaliths or rock art). And don't use or leave tea-lights in burial mounds, because the smoke from them can damage the stone. When I go into burial mounds, I don't use an electric torch - it heightens the atmosphere and your eyes adjust quite quickly anyway (there's usually enough light from the entrance).
One really good way to interact meaningfully with sacred sites is with the use of sound. Singing in caves and burial mounds is a really amazing experience. Most burial mounds resonate best with the male voice, but some caves (ones with pointy spaces in the roof) resonate well with the female voice.
» Heritage Action has a set of guidelines for visiting sacred sites