If you own or use a VHF marine radio, then it is a legal requirement to have an 'Authority to Operate', or to be supervised by someone who does. There are also several reasons for using a VHF as your primary form of safety communication:

Mobile 'phone networks are unreliable at sea

You can only can only call one number with a 'phone, and therefore surrounding vessels will be unaware of your circumstances

Modern DSC VHF radios can transmit position with distress message (when connected to GPS)

The rescue services can obtain a bearing of VHF signals

It is HM Coastguard's preferred method of communication.