I have the "feeling" that our engines works depending on charts based on the usage of X octane fuels, being X different for each kind of plane.
The Spitfires and Tempest are all restricted to 100 octane.
It would be nice to have the higher octane fuels, as they were in widespread use.
The German's developed MW-50 largely to overcome their poor fuels, for example the RAF tested a similar mix, but found that higher octane fuel did the job just as well.
In short, if you have MW-50 in a German aircraft, you have the effect you would get from high octane fuel anyway.
None of the British aircraft, at least, are modelled with high octane fuel.