Preach!

With some very specific exceptions, natural fibers are far better.

Give me a wool blanket or sweater (jumper in your terms I believe).

Wool is freakin magic.

My only exception is workout gear - the specialty synthetics are fantastic.

I second this. I'm almost always too hot and realized years ago that polyester fabrics made it worse. It's one of the reasons my ex and I used different blankets - I used a 100% cotton sheet, but he was the type of person who's usually cold, so he used fuzzy polyester blankets. He thought my blanket was too light, while I thought his was too hot.

The struggle to avoid polyester is the hardest part. I have a significant yarn collection for all my art projects, but over the past few years I've been trying to avoid polyesters (to avoid contributing to microplastics.) The only polyesters I bought since then were either made of recycled materials, or were special glow-in-the-dark kinds (because I haven't found cotton or bamboo yarn with that feature yet.)

Of synthetic, I can recommend rayon (that's made from bamboo), also known as viscose. While it is synthetic, it breathes wonderfully.