Hey there! Baby buns tend not to get the hang of litter training until neutered/spayed, because of hormones. But its not impossible, just takes a bit longer!
Cat litter can be highly toxic to bunnies. The scents, and most of them have baking powder/soda in it. Rabbits have super sensitive lungs and respiratory systems, even a change in scent affects them. Candles, air fresheners, wax melts, these all pose a risk for your bun. Cat litter is toxic if eaten as well, so I would highly suggest steering clear of all of them.
I use the non-scented CareFresh, its just plain recycled paper. I buy it because it keeps the smell in, it is super absorbent, and easy clean up. With just my one bunny, a couple handfuls will last him 3 days. With our bonded pair, it lasts about two days.
Even after the three days, it doesn’t soak through the bottom of his litter pan (depending on how heavy handed i am with it) so I don’t have to wash it out, and it just comes right out.
Although, the litter box I have is a plastic corner one with a metal grate on it so my bun (who loves to sit in his mess!) doesnt get urine all over him. I think it helps with not soaking through because there isnt constant pressure on the wet litter which would make it stick to the bottom.
Anywho, my suggestion is definitely carefresh! I live in the US, so hopefully its not super expensive in the UK. I glanced on Amazon.com and it didnt look like it was a lot. Just make sure its not scented! Best of luck!! 
