Earth-mode is a form of VLF communications open to all.
My understanding is that, in the UK, OFCOM does not require you to have a licence for such non-radiating systems. This is my interpretation and not a legal statement. If you are concerned seek your own legal advice.
Earth-mode is essentially conduction, mostly with the assistance of pipes and similar. In the past I have covered 6km by using utilities assisted earth-mode with just 5W.
This sort of experimentation is easy to do and at the frontiers of the hobby. I am sure far further would be possible with higher power.
See my earth-mode blog at https://sites.google.com/site/sub9khz/earthmode/vlf-xbm-blog