I was using Smart Cat litter for a while - it's wood pellets without anything added. I think the wood is cooked so it should reduce any natural phenols that you get in wood-based litters (but I've honestly no idea what an acceptable level is). You can buy it from Asda.
Merry started missing some fur on his hocks (which I think was probably because he kept choosing to sit in the rain to be honest), so the vet suggested I switch to something softer.
Biocatolet is pretty good - a bit expensive. You can get it from PAH and Sainsburys. Megazorb worked well too, but the huge bag was a bit of a pain.
I'm now getting the 23.5l Tumblefresh - it's around £17 but lasts for ages. It's definitely my favourite and the most absorbent of anything I've tried. There's a similar one called Carefresh which is is not as good, and the 60l bag seems a lot smaller than the 23.5l tumblefresh bag.