as I was asked about this stuff today in our temporary advice shop, I thought I'd list what I'd found out. If there's any errors, I'd be happy to take feedback.

Vermiculite - is a natural mineral. Hydrated, Laminar, Magnesium - Aluminium - Iron Silicate. Commercially mined in USA, South America, Africa and Australia. See : www.vermiculite.org

Supplied into the UK by Hoben International Ltd. It's used for lots and lots of different purposes and one is loft insulation.
Data Sheet

Micafil vermiculite is a naturally occurring mineral expanded at high temperature to produce aerated particles - 90% air by volume.

Asbestos confusion. While modern products are now sampled and checked, there's a risk of contamination but that too can be said of many other materials that are extracted from the ground. It's not as if it's hidden under the ground in a plastic bag ready to be delivered to your door. It needs excavated, graded, sorted from other rock types etc.

Is it any good ? Well yes and no. I think yes, as it's easy to work with, generaly non-toxic and can fill voids very efficiently. I'd be inclined to say no, as it's really only good for flat surfaces and will tend to pour down angled roof voids if not careful.

If you already have the stuff. I'd recommend you get it checked to make sure it's safe, and samples need taken from near the bottom and not the top as asbestos tends to sink.

Looking at the thermal benefits, it's not wildly different from other products that are generally available at local DIY stores/builders outlets.

Filling Vermiculite to the top of the timber joists isn't really sufficient depth, so you might prefer to choose alternatives.