You will need harness with chest and /hand/foot ascender to get back out! like above get some public liability insurance and you will be able to obtain keys to access the gated mines. Recommend an SRT training course if not familiar with frogging up a rope as its much more difficult than it looks, technique over strength and stamina...
Many mines in wales which aren't gated and don't need SRT to get around most of the workings, slate mine scales in wales are incredible, some are like something out of jurassic park with internal open top caverns lost from the outside world. Well worth a long weekend around snowdonia national park.