First to pick a couple of nits: ancient tin mining in Britain was during a period before the term "England" had a meaning; and - to save you from an angry Cornish mob - tin mining occurred in Cornwall, which most Cornish regard as distinct from England. Tin mining was important in Britain because of the Cornish reserves (although the same lode - a mineral-bearing vein in Cornish dialect - manifests in the southwest of Ireland as well).

Early mining in Cornwall took two forms: the first and easiest was conducted by what were called "steamers." These miners worked the streams of Cornwall to pick up tin ore that had been washed and exposed by action of the water. Some digging could occur with a process not unlike what the earlier '49ers employed in the California Gold Rush.

There was also some subsurface mining. Archaeologists have occasionally exposed these old works and found antler picks and limited timbers to serve as structural supports (the same sort of technology was used by flint miners elsewhere in Britain). Dewatering was a constant problem that was largely unaddressed, so depth was limited. Work underground wasn't safe, but it was likely not more dangerous than what occurred in the pre-industrial and industrial periods. I find the depiction of mining in the recent BBC/PBS Poldark series to be satisfying for that later period. That level of technology began to settle in during the late middle ages and was perfected over the centuries by a series of technological advancements. During the nineteenth century the industry settled into a form that could be regarded as "modern" underground mining.