Don't forget that you're not obliged to buy Hills (or any other brand) from the vet - you can find it cheaper online (try zooplus.com or vetshop.co.uk).
One of our 17-year olds has Royal Canin Senior Consult Stage 2 wet food which seems to be really good for her coat. There's a Stage 1 for slightly younger cats, and there's a dry version.