England's best and almost only fighter in 1939 was the Hawker Hurricane and the predominant Battle of Britain fighter was the Hurricane, fortunately ordered very late in the day by the government, but concieved from the outset as a fighter.
The Spitfire was a shoestring development of a privately sponsered seaplane in 1931.
Boeing had the advantage in 1940 of building on the example of the Hurricane, Spitfire and the Messerschmitt with the aid of full-sized wind tunnel testing, and the luxury of considerable state funding without the preasure of being at war.
As the American war effort was uninterupted by bombing, they were able to perfect the Mustang before the Italian, Pacific and Normandy campainges, by which time its aerodynamics, maintenance, reliability, endurance and refinement and sheer numbers made it second to none as the most effective fighter.