When viewed in context the picture is clearer. Other units and stations at the time were clearly doing quite a bit more than "tested" 100 octane:
151 Squadron
611 Squadron
74 Squadron
605 Squadron left Hawkinge on 28 May 1940 where 100 octane was obviously present enroute to Drem, also which obviously had 100 octane. For some reason they stopped over at
Shawbury, which I understand was a Flying Training School and apparently didn't stock 100 octane ("unavailable"). It's interesting that it was determined that the Squadron was unable to proceed further without the 100 octane fuel. This suggests that once having converted to 100 octane, whatever fuel the flying school was using (87 octane?) was unsuitable.
610 Squadron Spitfire Mk I at Hawkinge in early July 1940. Note the petrol bowser marked for 100 octane fuel.
100 octane was required at Drem, 605 Squadron's destination, as well as Debden where 504 Squadron was based.
Obviously Drem was stocked with 100 octane fuel by February 1940
602 Squadron
Fighter Station, With the Spitfires in Scotland. Flight, No. 1631 Vol. XXXVII, March 28, 1940 (602 Squadron at Drem)
111 Squadron
