I've had a little rummage around the internet generally and scientific google in particular, and it looks like z. jujube and z. spina-christi are either the same plant, or very closely related plants of the same genus, in the same way that the different forms of cassia - senna alexandrina and senna italica - are very similar and are used in exactly the same way to condition hair. (Just for clarity and to avoid creating yet more confusion by using cassia as an example: although the various sennas have different levels of yellow dye, ziziphus is colour-free, so even if it's not exactly the same plant, the difference wouldn't have an effect on hair colour.)
The common name for Ziziphus spina-christi is Christ’s Thorn Jujube, and I suspect the non-English sites that call it Ziziphus Jujube just conflated the two versions of the name. Jujube is more often used when talking about the fruit of the tree, but all of the references that I found are to genus Ziziphus/family Rhamnaceae. So ordering your sidr for hair at either the UK or Italian shops should give you the same stuff
Bookmarks