There has been some debate today on the RSGB's LF Reflector about how legal amateur transmissions in the UK would be at frequencies below 9kHz. Apparently the old Radio Communication Agency failed to come back with an answer to a request around 2000-2001. We're wondering what OFCOM would say today, so I've emailed them to ask. They have already replied and told me the query is being forwarded to their Spectrum Management Team and they are considering their answer over the next few weeks.
Communication over any distance at frequencies below 9kHz is a real challenge and the modes used are usually conduction through the earth or sea (so called "earth mode") or by induction. Very little power is radiated unless enormous powers are used e.g. in the USA's Project Sanguine when megawatts were used at 76Hz to communicate worldwide with submerged submarines. Distances much beyond 10kms are highly unlikely with amateur powers and systems.