This is the horrendous act? Submitting a report of alleged offences to the relevant authorities for consideration? Not the actual alleged offences? Do they not reach a threshold for investigation in your opinion? Why not?
If a UK citizen is alleged to have committed offences, there are processes for investigating concerns and for legal consequences if needed. Regardless of the name of the military or paramilitary force in which they're fighting, if a UK citizen is alleged to have committed a war crime, that should be investigated. The alternative is risking having people who have done such crimes walking free in society and presenting an ongoing risk within society without facing justice for their wrongdoing. If the threshold for further proceedings is met, then I hope those proceedings can be undertaken justly, to give fair hearings to all parties. If people are responsible for offences, I hope they face justice within the framework of UK law. If people are innocent, I hope they can also find justice and closure in having their names cleared.