DON’T FORGET YOUR LEGS!
You’d be surprised how many people cover their upper body with super-warm gear and then sacrifice their legs to the cold. Although they’ll be working and creating heat, cold legs can lower your blood temperature enough that your jacket becomes irrelevant.
Consider some wicking underwear to keep your groin (which has a lot of blood vessels close to the surface and gives off a lot of heat) dry and warm. And tights or thin leggings under walking trousers can make a real difference to your body temperature over a day.