Bulged canopy not because the Oxbridge types were that tall, rather big-headed.
In August 1938, K9789 & K9792 were alloctad to 19 & 66 Sq for operational trials.
among the complaints
inadequate starter,
engine leaking oil,
u/c pumping and
low cockpit canopy (yes, because of taller pilots hitting their head).
On 4 May 1939 there was a Press Day at Duxford and there is some pics of a row of 19 Sq Spitfires
WZ° (no Sq letter) bulged canopy with unpainted frame
WZ°C low canopy
WZ°(I or L) bulged canopy
WZ°(H or U) bulged canopy
At this date UAS were probably flying open-cockpit aircrafts and their members were called-up for active service not earlier than September 1939.