Hi Julie
If you do want some prescription food for renal problems you can but it here
K/D Diet Canine & Feline : Vet UK for Healthy Happy Pets without a prescription but it would be best to make sure from your vet which one he thinks is the most suitable. The real key is a low protein diet.
Make an appointment for a good chat with a senior vet in your practice. They are usually less alarmist than the younger more keen vets and he may suggest some ongoing medication that you coul give your cat to support his kidneys, if the vet feels there is a need.
Marcia.