Infuriatingly, the short answer to the question is: "It depends".  It depends on whether the employer supplied you with a cell phone in order for you to do your work and it is the contents of that cell phone that the employer is looking at.  In terms of our legislation, particularly the Regulation of Interception of Communications Act 2002, citizens are generally protected against having their private communications monitored by the State or other people.  However, as with all legislation, there are exceptions.  One exception is when a person has consented to the surveillance of their data.  Often, an employer will provide an employee with a laptop or a cell phone and, in its policies, the employer will state that the employee cannot have an expectation of privacy when using equipment provided by the employer.  Even though this places limits on an employee's right to privacy, it would not be viewed as illegal as the equipment has been provided solely for work purposes.  If, however, the employer has hacked your personal cell phone that you do not use for work purposes and has not been supplied to you by the employer, then you may have an argument that they have contravened the law and intercepted your communications illegally.  This may not be enough to save your job if you have been "bad mouthing" your employer to others, as that may indicate that there is a break down in the employment relationship.