Just an update - first time today using it just over a t-shirt with no top layer. Soooo well ventilated, it was basically like wearing a t-shirt when you're riding a bicycle.
Tiny downside / advice - I got the Utility version as I thought the pockets might come in handy. Thing is, the pocket bags are cotton canvas or twill with no stretch so they're a pinch point if you get the jacket with a tight fit.
I unpicked the pockets.