American Express offers in other markets are irrelevant. What UK-issued cards offer no foreign transaction fees -and- compelling rewards?
Literally any rewards visa or MasterCard with Curve will allow you to spend abroad with 0% FX and still get the rewards. I get your point though, the UK market doesn't need it. But once the whole Monzo etc card calms down and 0% becomes the norm, people will start to look for other benefits.