Not sure I can advise really
I am on the committee for the RSPCA so don't really have anything to do with this side of things although we deal with some of the requests for money. What RSPCA are you dealing with? The problem being and I know this is wrong but the way they would look at it is purely down to expense. £600 on one cat could go a long way to helping many others and if they allocated this money to every cat in their care, hundreds would suffer because of it, this is the way they would look at it, not me
It's a sad situation all round I'm afraid. If it was me, and I could afford it and the cat has a good quality of life and will do after the op, I would go for it and deal with them afterwards. The other thing that may be worth suggesting although you shouldn't have to, is to offer to pay for some of his treatment if they can pay the rest
but I would only suggest this when all other avenues have been exhausted as you really shouldn't be paying out for a cat that you are basically looking after. It may also be worth getting it in writing from the vet to say this procedure is 99% effective and will solve all his problems. This would be another issue, they will probably be looking at it long term and as long as they know it's money well spent in that it would solve his problems, they may go for it. I hope something can be sorted out and I will try and get some more info on how the assisted adoption programme works.