Last week I took my kids for a walk in the Lake district. Kept it simple, as I have an ankle injury from many years ago and knew I couldn't cope with a big walk.
So we went up Scafell Pike via Mickledore, down via the guides route and face of great gable.
The weather was a treat, blue sunshine for most of it. You could see Scotland from the top.
Quite a few walkers were in jeans. No problem. It's sunny weather.
By 5pm, the cloud base dropped below 600m (Scafell is +900m). Winds picked up to 60mph. It stayed like that for 3 days, except for when the clouds dropped to 300m.
Now, those people clad in jeans and cotton shirts would have been rapidly soaked on the tops. Fine if they could walk off quickly. Not fine if they struggled to navigate.
Read the local mountain rescue reports. See how many reports can be summarised thus: "Became disorientated in cloud. Wearing jeans and cotton shirt. Hypothermic. Escorted/stretchered off."
If you want to know what not to wear on the UK hills, ask the local mountain rescue. Top of their 'no' list will be jeans and cotton shirts. That's not because they bear an unnatural hatred of denim, it's because of the number of times they have to rescue someone who gets hypothermic once their cotton clothes get soaked in the cloud and drizzle.