Orikoru
Tour Winner
Here's the link since I neglected to include that before: https://www.uniqlo.com/uk/en/product/ultra-light-down-vest-452739.html?dwvar_452739_color=COL69I have a Uni Qlo one that's light as feather and feels nice to wear. It packs down really small as well so easy to chuck in your bag if you take it off. Pretty sure it's water resistant. Cost £40.
It does say it's "water-repellent" there. I didn't think there was much point in getting a fully waterproof one since it doesn't give full coverage anyway. I really like the fact that it's not too thick and padded, because I've had ones like that before and found them difficult to swing a club in.