Hi
Sounds like vermiculite or perlite and you can get it from all garden centres B&q etc have a look at the link this gives you a explanation to what it is
http://www.thegardensuperstore.co.uk/acatalog/Vermiculite_and_Perlite.html

I think she (Carol Klein that is) does actually use grit or gravel - have a look at the garden centre, they should sell small bags of horticultural grit or gravel. And I mean small, hand sized.

Failing that, grit for budgies, from the pet store, will also work, though it has more lime--sometimes a boon, sometimes a bane.

I wondered about the grit they sell in pet shops to go in fish tanks - it's always dusty though, and I'm not sure what that is. I've used it after washing it thoroughly first.

I have tried both grit/gravel,and Vermiculite...didn't find any difference in germination results...although I do prefer grit/gravel to mix in with compost in pots,for drainage..

If you have a local pet shop that sells things in bags and they sell poultry supplies, Hen grit works out cheaper than hort grit. especially if you can buy it by weight.