Cex has no Vardiff implemented. You can manually change it in the Workers page.
I can't speak for BTC but it does have some form of Vardiff on the LTC pool. When I boot a 1.2MH rig onto the pool it starts at 16 but settles at 64 after a few min.
/edit Or were you referring to the leased mining option?
No lease option. Just my own two rigs. Maybe I should set it to 0 in the workers
Page and let it do it's thing. I had 2.7kh/s rig at 500 and 1.9kh/a rig at 275.