Over here across the pond the number on the pump is the average of research and motor octanes or (RON+MON)/2. The spread is usually 6 to 8 points so the 91 that we see in California and Nevada should be 94 or 95 RON. Not all that different than the 97 that you are able to get.
How hard do you run your XR? I'm guessing that you are mostly on the pavement in the UK and the temperatures are a lot less than we see in the desert southwest. Temps over 40C are not unusual. That makes pinging a lot more likely.
My XRL is used on the road race tracks only, so its gets used very hard with minimal cooling (its in a beam frame, which gets too hot to touch!). I run an Aprilia RSV oil cooler to help out.
The engine has had lots of lubrication issues shall we say!!