I tried one from savers for a pound/2 pounds, and it was fantastic, really covered the greys for ages (I find the colour fades rather than its regrowth that needs doing, even though I use perm. dye) but I use black so no issue over the colour.
I think it's been discontinued OP. I was using the dark brown, the only one I had found to cover my greys really well and not drip all over my bathroom. I paid £10 for a box from Amazon a few months ago but can't find it again
I use L'Oreal Excellence Cream in 9.1 (light ash blonde I think?) and people are surprised when they hear that it's a home dye job, not salon. Never had any colouring disasters with that brand/shade and my hair stays in good condition.
When i went to their page they didn't have 9/0, five other shades but not that one. No idea why you see it and I don't . Shame though. Would have been ideal. Nearest Morriston is about 25 miles from me but I could have got there.