Hi Ross
SpaceOAR gel insert is definately available on the NHS. I know because I had one fitted 9 weeks ago. So I'm not sure what the criteria is for Oncologist approval of this procedure. Mine just told me I'm having it.
The procedure itself is very similar to having a Biopsy: legs up in stirrups, local anesthetic, needle goes through the perinium, gel then injected between the prostate and rectum. No significant pain or discomfort. In the room for less than an hour.
The idea is to put a physical barrier between the prostate and the rectum so that the rectum has (some) protection during radiotherapy. The gel dissolves over a period of several months.
Two weeks into radiotherapy I developed mild diarrhea, but not too bad really. Who knows just how bad it would have been without the spaceOAR insert, there are some horror stories here.
I hope you get what you want, and from experience I can recommend Stokes Centre for Urology Guildford if you decide to go down the spaceOAR private route, although I've no idea what they charge privately.