It is entirely possible for a practice to be unfair but for it not to be illegal and this is precisely the case here. It is quite common to believe that there is a right to equal pay across the workforce, especially for workers who perform the same or largely similar jobs. However, the law on equal pay was actually introduced to specifically deal with pay inequalities between men and women and, specifically, to ensure that women were not paid less than their male counterparts just because of their gender. That is why an equal pay challenge would only be a relevant argument if the reasons for the difference in pay were gender-related.
Therefore, if the difference in pay is not because of a worker’s gender, in reality employers are free to pay their employees whatever they deem appropriate, as long as it is above the current National Minimum Wage and in accordance with the employee's contract of employment. It is actually not unlawful to pay employees doing the same or similar jobs different rates. As mentioned, this would only be an issue if there is a gender-related reason behind this.
In summary, unless there were clear gender-related reasons for the difference in pay, which cannot be justified by any of the above reasons, there is nothing illegal in paying different workers different rates of pay, even if they are performing the same job. In these circumstances, the only argument would be on ethical grounds, rather than legal ones and it is only legal breaches that can be formally taken further.