BP ultimate isnt 99ron, in fact its not much different from any other supper unleaded, just has the added engine cleaners...its very good for cars that run on 95ron as most uk cars do..in that case its the best.
But if you look in your handbook, and the sticker inside the petrol cap cover it recommends that you use 98ron, in France 98ron is their standard unleaded and so French cars are set up to run on this..
BP will not harm your 172 and itll be fine, but on Otimax itll be more than fine....I can tell the difference, I tried BP expecting it to be as good as Optimax (so its not just in my head!) and was disappointed, Optimax makes a noticeable difference in my opinion!!
I hope this helps and I’m sure someone else will come along and explain it in more technical terms