You need to get him to a vet asap and
Blue Cross sorry meant PDSA may help you but they do things by using cheepest option and if you have £500 pounds of cover I would see a regular vet but ask him for an estimate first.
Probaly will do some tests and then if kidney probs prescribe meds and special food...........the food you can possibly buy cheaper elseware and sometimes cats wont eat it anyway.
E&L are not known as a very good company so maybe a good idea to ring them and makes sure what your cover is, get a claim form and ask them how long it takes for them to pay out and will they pay the vet direct.
Good luck and loads of peeps her can advise about food and cheapest options for meds and food for cats with kidney problems