jb2_86_uk: It is easy to be misled by what appear to be facts in print.
The question of instrument inaccuracies might have applied later but at the time of Sptifire dog fights in WW2 there were no mach indicators, the ASI's were calibrated in MPH and, IIRC, only went to about 550.
I also think one would have been hard pressed to find an RAF pilot at that time who even knew what the speed of sound is, let alone the effect of compressibility.