Everyone can be liable for assault, including police officers and security guards.
Police have powers that allow them to handle people more than anyone else. A security guard can act using reasonable force to prevent a crime from occurring, the same way that any member of the public can, but for security guards courts will often deem that they are allowed to act more forcefully than the average member of the public.
Licenses: Anybody employed by a private security company, agency, or contractor must be licensed by the Security Agency Industry (SIA). A ‘Security Licence’ is needed for those guarding premises against unauthorised access, occupation, against outbreaks of disorder or damage, and against theft. This includes “providing a physical presence, or carrying out any form of patrol or surveillance”. However, if the activity is performed by a direct employee of a company (i.e. is employed ‘in-house’), a licence is not required.
Front-line staff MUST wear their licence (credit-card-sized) where it can be seen, unless wearing the licence would undermine their role (i.e. store detectives). Failure to do so is a breach of the licence conditions, and may be reported to the SIA. Non-front line staff (i.e. managers) must carry their licence (a letter) with them. Both groups of staff MUST show their licence when asked by a police officer or an employee of the SIA (but, not by a member of the public).