The UK targeted and killed some British supporters of PISSI, in Iraq. Is this a sensible strategy?

Was it legitimate to kill them with a missile? In practice, PISSI and the UK are fighting a war, and in those circumstances it is morally justified for the UK to kill PISSI fighters. Were these men PISSI fighters? The articles I have seen have not given that information.

The UK government says that they were talking about committing crimes in the UK. However, that's not the same as waging war, and is not grounds for killing them in Iraq.

It seems they did not even have concrete plans. If they were talking in a vague way about committing crimes in the UK at some indefinite time in the future, that shouldn't even be a crime until they make it concrete.