Gill, you can get other renal dry foods online that work out a lot cheaper - I get Zi Purina NF, it is £11.06 a bag, and even though they added p & p on (cos like you, dont want to spend £39 to get free p & p), it still worked out about £2 cheaper than getting RC from my vet - I know £2 isn't a lot, but it is most of the price of a 6 pack of wet for Buster. I can send you some if you would like to try it first, and also some Eukanaba Renal? If I remember rightly, Best Pet did a better deal on Purina than VetUK, as their postage charge was less, so they could be worth looking into as well.
Prescriptions for some items work out a lot cheaper - Sam's Vidalta were £10 a month cheaper online, and the vet gave me a 3 month repeat prescription, so my £5ish prescription more than paid for itself.