The Biotherm product I'm telling you about is in a orange tube with a white lid and blue lettering in a 75mL format. You should use it year round. Here in Canada we have such shitty winters that you're practically covered from head to toe because of the cold. As a result I don't really use it in the winter. It might be a different story in the UK.
It's never too late to start using sun protection.
And remember to use sunscreen only in the morning, and a light moisturiser at night.
You can't compare two cleaning/conditioning agents which are designed to be washed away from the hair with a product stays on the skin. Next thing you know people are going to be saying "what's better? summer and winter tires or just all seasons." It's not the same thing.
Unless of course you use crap products, in which case you'll want to use two different products.
I really don't think that's a huge concern. I'd be more concerned with the sun than those nanoparticles. Whenever they test these things on animals they expose them to quantities that are much higher than a human would ever be exposed to. They probably suffocated the poor animal. Dirt particals will do that...