There is no doubt that IS is a heartless enemy which blows up, beheads and shoots mostly innocent people who somehow violate its theology, often by no more than just existing. So there is a certain logic in saying we need to fall to its level to beat it. But this reasoning is wrong. First because any terrorist organisation is doomed to fail in its mission, because although all cause mayhem and suffering, none have succeeded in their aims. Second because to stoop to the terrorist agenda is in itself a victory for terror. Finally it produces false intelligence from half crazed victims, which in turn leads to wrong decisions and bigger disasters.
May made that clear to Trump. He showed some ingenuity by continuing to support water boarding, but deferring to his Secretary of Defense, James Mattis, who opposes it. But for how long? The UK government needs to keep an eye on this. For the British people of all political persuasions or none, it is a red line.