If you look at the stockpile amount of 100 octane fuel, it actually goes up between March 1940 and October 1940! That could only mean either Britain wasn't using the fuel and was saving it. Or there was plenty to go around.
Personally I believe there was more than enough to go round and let's face it Britain was fighting for her existence and wouldn't hold anything back.
Oh yes and not forgetting the order for squadrons to make the necessary alterations to their Merlins so their Hurricanes and Spitfires can run 100 octane fuel.
http://www.wwiiaircraftperformance.org/ap1590b.jpg