Alex.
The mine you are referring to, was first called Dover Colliery and the Shakespear Colliery but it was originally the drift boring site for the first channel tunnel. That was abandoned because the government at that time decided that if it was finished, it would be too easy for the French to invade England.
The owner new that coal had been found in Kent so he turned his boring machine and started boring down towards coal.
The story goes that he did find some coal but when trying to get financial backing; he secretly took a load of coal down the pit the day before his campaign to get funding and brought it back up in front of potential backers.
Prone to flooding with several lives lost, it closed.
There is a different story that someone on here sent me details of that showed that he it did produce coal but it still did not last for long anyway.