Unless someone works in a job where their terms and conditions of service are laid down by statute (e.g. teaching) or where a trade union collectively negotiates a common deal for all staff, every employee's contract with their employer is entirely separate from that of any other employee.
For example, your employer could employer someone to work alongside you, doing exactly the same job, but pay them 10 times as much as you get, give them ten times as much holiday and pay them early at holiday times while making you wait until the normal date. As long as the employer stays within the laws relating to minimum pay, minimum holidays and various specific forms of discrimination, it's all perfectly legal.
In the major City financial institutions, people work alongside colleagues (doing exactly the same job) who're paid 4 or 5 times what they're getting. As stated, it's perfectly legal, and long may it remain so.
Chris