in the UK and most other jurisdictions, they can legally tell you that, but cannot enforce it in any way
Under the National Labor Relations Act (NLRA or the Act), employees have the right to communicate with their coworkers about their wages, as well as with labor organizations, worker centers, the media, and the public. Wages are a vital term and condition of employment, and discussions of wages are often preliminary to organizing or other actions for mutual aid or protection.