IIRC Mason - in British Fighters- reckoned the Battle & the Defiant were there for two reasons - initially because they replaced biplanes with similar crew configuration so it was replace like with like - and secondly because they were all that they had to put through the "shadow" factories to start with. The Spitfire was too complex for people starting out
In my opinion it was criminal for the Air Staff to send out aircraft in 1940 without a front gun and only 4 in a turret when as early as 1934 they'd agreed the future fighter (s) would need 8 forward firing guns