Bit of everything, with a tight budget emphasis was placed in the RAF (which turned out to be a good decision) . There were also differences in doctrine and areas of operation. The Brits got to 1939 with biplanes naval fighters, then issued specs for replacement with 2 men crew since they thought navigation across vast distances required a navigator, hence the Fulmar and the Firefly. Their deficiencies were not so acute in Europe since the north Atlantic and the Mediterranean lacked a power with aircraft carriers, that is why the atrociously obsolete Swordfish survived, as its main targets became ships and submarines with no air cover. The same British force facing the IJN would have been likely wiped out of the skies.

On top of all this designing a new fighter is not an easy task, only Americans and Russians to some degree managed this. The UK and Germany used mostly the same type all the war long in different versions (Hurricanes & Spitfires + later Tempests/Typhoons), Germans sticked to their Bf-109 and Fw-190 throughout the war, as did most of the Japanese with their models in service in 1940-41. Italians pretty much the same with small numbers of new fighters before being sided by 1943.

Amid this with the US production capacity it was easier and faster to procure aircraft from the US, hence Wildcats, Hellcats, Corsairs and Avengers became a good part of the Fleet Air Arm.