with regards to sft42's examples: 24hr garage attendants DO have the system i illustrated (i believe), milkman don't fall into this category as they are "out-and-about" and can summon assistance immediately, breadmen/bakery workers either are not working alone or are employers of a one-man operation - the legislation only applies to employer/employee situations, the onus being on the employer to provide adequate security and safety for his/her employees.
On call tradesmen?