The challenge with Scottish climate is high moisture content even at sub zero temperatures.
Go to the alps or Scandinavia when its well below, the air is very dry so all that moisture that you expire is transported out through your clothing so you remain dry even when working hard. If you do get sweaty a simple opening of a jacket and you are quickly dry.
In Scotland the air is already full of moisture so you can easily soaked under neath your rain gear, and because there is little gradient in vapour content between under your coat and outside you remain damp, and thus you can get cold very quickly.
As stalkers on the open hill, we make matters worse as you often have to crawl through wet grass, heather, bog etc.
Choice of clothing and especially underlayers is vital. For me only one option and that’s Merino wool. It remains warm even when wet. It’s also worth carrying inside a dry bag / vaccuum sealed back a dry pair of socks and a dry base layer top and a lightweight down jacket. If you have a big sweaty climb in the morning you can then strip off and put on the warmth on the tops.
Key I do think is having the ability to provide yourself shelter in case of difficulty. We can all get the weather wrong, or twist an ankle. Having a bivvy bag / survival bag / poncho in your pack just gets you out of the wind and rain and into the dry. They weigh little but can save your life.
We also seem to want our waterproof layer to be sleek fitting, but I can’t help feeling a poncho type thing keeps the water away from you whilst providing plenty of ventilation. But in strong wind they are a pain. A poncho can also act as shelter, especially when you are forced to wait.