From what I have read about the whole Battle of France period the British leadership was being tugged between the desire to support operations on the continent, and the desire to preserve sufficient numbers of aircraft on home soil to defend the island if France fell.
So you'd like to have more aircraft defending the troops coming out of Dunkirk, and you'd like to answer the desperate pleas of the French to supply more squadrons... but you're worried about leaving the home defences too thin if (as seems increasingly likely) France is lost.
I recall John Terraine's history of the RAF is good on this.
History would seem to show that our leaders got it just about right. Air cover over Dunkirk was as much as could be spared, and we scraped through the Battle of Britain by the skin of our teeth.
Side point - I seem to recall that the Defiants did very well over Dunkirk, their first true combat test. But when the Germans worked out that they had no forward-firing armament things changed.
I once met a Hurricane pilot who was over Dunkirk and said he saw the bomb that went down the funnel of the Lancastria.