…That Caesar invaded Britain (or tried to) at least in part to get access tin deposits? A generation or more later Claudius ordered a full scale invasion in 43 AD. That one stuck. One of the attractions that time was lead. Romans were heavy users of metals of all kinds, and as they spread out across Britain, they established mines. By 70 AD Britain was the primary supplier of lead and silver to the Empire.
The second major commodity was iron. The Ordinance Survey Map of Roman Britain lists thirty-three iron mines. They eventually found their tin, largely in Cornwall.
I came upon this subject researching mining in Wales for The Defiant Daughter. The Romans had control of Wales—and began mining—by 74 AD at Dolaucothi. They were looking for gold. Their initial efforts made use of the famous Roman engineering and hydraulic mining. They built reservoirs in the mountains and a seven mile long aqueduct. When the water was released in a rush it brought down the topsoil and plants, scraped off the entire surface layer and exposed bedrock and a seam of gold underneath. They worked Dolaucothi until at least 300 AD with opencast and finally deep pit mining replacing hydraulics.
What about coal? We know they used it, but it appears to have been so abundant it was extracted from the surface, unlike the characters in my story who have to go down to the depths to get it. One suspects coal had much less value to the Romans than it did when it fueled the industrial revolution.
https://romanobritain.org/11_work/raw_mining.php
http://earthsci.org/mineral/mindep/ancient_mine/deep-vein_mining.html#Types