First time buyer - Lorette sizing and warmth for London?

Hi! I’ve decided to buy a Canada Goose Lorette and want to make sure I’m buying the right size, and that it’ll be sufficient for London winter.

I was only able to try on an XS in store, but am now wondering if an XXS might be better (I’m a woman on the more petite side, 5’6, 31 chest, 24 waist, 35 hip). I tend to buy the smallest size available in most things, but I want to make sure the coat won’t be too tight / restrict my arm movement. No stores nearby stock the XXS so I can’t verify on my own.

I’m hoping this style will work for London weather, where winter is ~35°F/1.7°C at the very coldest. It may snow lightly but mostly it’s just rainy.

Any advice is greatly appreciated - thank you so much!