Simply put, as I understand it from reading the various threads on this forum plus the threads on ww2aircraft.net
linked by Klem on page 9 of the Spit MK I/II thread, RAF fighter command's Spitfires and Hurricanes flew combat sorties using 100 Octane fuel during the Battle of Britain - there is ample and overwhelming evidence for this backed up by references linked; there has been no evidence provided backed up by a reference for the use of 87 Octane fuel by Spitfires or Hurricanes during BoB. So simply put, in a combat flight sim of BoB, Spitfire and Hurricanes should be modeled using 100 Octane fuel.