If a person is present and does not give consent, the security officers do not have the authority to forcefully search a person's property. On the other hand, they have the legal authority to search property that has been left unattended in questionable circumstances. They can also search an unconscious person's property if they are attempting to identify the unconscious individual and it is in the best interests of that person's welfare.
Security guards do not have any more legal rights than any other member of the public; they are just hired by a company to assist protect it from theft. Under certain circumstances, security guards, like any other member of the public, can make a citizen's arrest.
You can make an arrest for most indictable offenses (including either way offenses) under section 24A of the Police and Criminal Evidence Act 1984 if:
The only way an arrest can be legal is if: