When teaching radio operator courses I do not remember any warnings about safe distance from a transmitting VHF antenna, but there were serious warnings to keep away for transmitting MF/HF antennas. Perhaps someone can correct me regarding VHF antenna safety if need be. The base of my stern antenna is about four metres above the deck so I am not too worried even if I had to transmit at full VHF power.
This is a relatively new requirement that post dates both my teaching and issue of licences. Like many regulations its purpose seems to be more to protect the rule makers.As part of the Ofcom license process, you the licensee have to calculate your safe operating distance and are meant to enforce it.
An RF burn is a nasty experience, they have a habit of burning out nerves,.are deep and often get infected.