I lost Gizmo, my first cat to kidney failure; he was 19 though. Cats don’t show symptoms (excessive urination) until they have lost about 2/3 of their kidney capacity according to my vet.
I currently have another cat, Frank, that was in kidney failure about a year ago. He’s pretty much normal, now. (I had blood work done and one of his kidney parameters is at the top of the normal range, the others are well within the normal range) Unlike yours, he always used the litter box, though.
What helped him was kidney specific prescription cat food:
http://www.hillspet.com/en/us/products/pd-feline-kd-dry
http://www.hillspet.com/en/us/products/pd-feline-kd-chicken-canned
Vet prefers the canned food as it helps keep the cat hydrated better. Frank is not the brightest, and hates the (any!) canned food, but likes the dry food a lot. So he’s doing well at present. The food isn’t cheap, but well worth it IMO. I really can’t say enough about how well the food worked for him. YMMV.
I do realize that his kidneys are probably still the weak link in his health, and that they’ll fail at some point in the future, but he’s doing well for now.
I’m in the US, so that particular brand may not be available in the UK, but I’ll bet that there is something similar available.
HTH