The Public Interest Disclosure Act 1998 was brought about as a result of a number of high profile disasters, following which it was discovered that in many cases employees had been aware of risks associated with the conduct of the particular business but had felt intimidated into not speaking up. As a result of this the government brought into effect legislation protecting those who 'blow the whistle' on errant employers who fail to comply with health and safety practices that result in injury or death. However, they went well beyond that and offered protection for those who make complaints in relation to other unlawful acts such as fraud, damage to the environment, and other miscarriages of justice.
Significantly, where a worker can argue that their dismissal was due to them having raised a 'whistleblowing' complaint, a court can award unlimited compensation. Also an employer cannot contract out of the right to bring a whistleblowing complaint, and bear in mind the right is not just limited to employees, e.g if you employ an agency driver they are protected in the same way. Rather like discrimination law, the right to bring a claim arises from the first day of employment.