You will hear reference to the Disability Discrimination Act (DDA) and disability discrimination legislation, though in fact in 2010 the Equality Act 2010 replaced most of the DDA.
The Equality Act 2010 still aims of course to protect disabled people and prevent disability discrimination. It provides the legal rights and obligations in the area of employment that you need to be aware of, as well as education, access, property, and the functions of public bodies.
Visual impairment is explicitly covered by the Act.
Through the link above and also this one to the Home Office pages on the subject, you can learn more about the Act and about your obligations as an employer. However, in summary, you cannot discriminate against (treat differently) a visually impaired staff member because of their disability, and you must take reasonable measures or make reasonable adjustments to ensure that he or she can continue to perform their duties as an employee.