UK octane rating <> US octane rating

Snip-it
Jaguar V12
Fuel Requirements
Just determining what fuel is recommended for use in the Jaguar V12 engine is a bit of a challenge. For example, in the XJ-S H.E. Handbook for the US-spec 1983 model year, on page 111 it merely lists a "Recommended minimum octane rating" of 91 (RON) Lead Free. Many people have read this and concluded that the car calls for Premium, since the typical octane ratings in the U.S. are 87/89/91+ -- but that's because they fail to notice the reference to RON. US gasolines are not rated in RON; they are rated in the average of RON and MON, commonly marked on the pump as (R+M)/2. Unless you know how to convert RON to (R+M)/2, the spec in that handbook is meaningless.

Apparently, Jaguar improved their handbooks later. Douglas Dahl says, "It states for my '89 H.E. with Lucas ignition 'In the United States, Federal Law requires that gasoline octane ratings be posted on the pumps. The octane rating shown is an average of Research Octane Number (RON) and Motor Octane Number (MON). This number is also referred to as the anti-knock index (AKI) or Average Octane Number (AON).

"'Unleaded gasoline is available in two grades; 'Regular' labelled at the pumps as 87 AKI (91 RON) and 'Premium' labelled at the pumps as 91 AKI (95 RON). Use unleaded gasoline with an octane rating of 91 RON, i.e. Regular fuel.'" Apparently Jaguar was unaware of the availability of mid-grade 89 octane fuels.
End Snip-it

Here's the web link

Here's the summary:
US 87 = UK 91 RON
US 89 = UK 93 RON
US 91 = UK 95 RON
I would guess that US 93 = 97 or 98 RON