I use Aspen litter for my gerbils that I got at Petsmart. Here is a link to Petsmart to show you.
petsmart aspen litter
When looking at the link click on the word Learn next to the photo to find out other options for bedding.
The Aspen bedding is what Petsmart used in their store and recommended on their adoption sheet for my gerbils.

Here is a photo of them with the Aspen litter in their cage.

I guess you probably don't have Petsmart stores in England but maybe you can find the litter somewhere or maybe order it online.