UK rates fuel in RON. USA uses RON+MON/2 or simply AKI.
UK RON 95 = USA 91 "octane"
UK RON 99 = USA 93-94 (Super Unleaded)

With that said. Unless you are getting knocking, no sence in running a higher octane fuel than what you are tuned for. If you have no mods and the car calls for 91... than that's what its designed to run on.

I'm sure there are a lot of other opinions on this topic, but hopefully Eurocharged will chime in with thier Tuning experience.