The EAT in International Petroleum Ltd v Osipov has made clear that an employee can bring a whistleblowing detriment claim against a fellow worker in relation to their actions in dismissing him (for which the employer may be vicariously liable), in addition to an unfair dismissal claim against the employer. The law only prohibits bringing a claim based on dismissal as a detriment claim against the employer, not against colleagues instrumental in the decision to dismiss.
Given that detriment claims have a lower standard of causation and the possibility of injury to feelings awards (not available for unfair dismissal claims), it may well be advantageous for claimants to bring both types of claim. The colleagues and the employer can be jointly and severally liable for the losses flowing from the dismissal, and the award against the colleagues can also be subject to the statutory uplift for a failure to comply with the Acas Code of Practice on Disciplinary and Grievance Procedures (presumably a tribunal is more likely to award this where the colleagues were responsible for the employer's compliance with the Code).
Employers may wish to update their whistleblowing training for managers to reflect the possibility of personal liability for post-dismissal losses.