yeah, my local branch are really excellent in every respect. They have slightly higher than average requirements too which is good
Have you thought about getting in touch with your local branch and seeing if you could use their vets?
I believe sometimes they will come to an arrangement whereby you pay cost price for anaesthetic, medications, etc. It doesn't cost them anything as they just pay the vet their usual salary and their aim as a charity is to help as many animals as possible so it's not like you're competition. Obviously this depends on the branch but it's worth asking.
I personally think you'd need to be making a significant saving going to a rescue over getting a bun from a free ad and neutering it or people will save themselves the hassles of home checks and do that. So I would say £40 for a buck and £60 for a doe is about maximum