You cannot need what you cant have. Lend Lease allowed the allies to prosecute a different war. British production was top heavy with 4 engined bombers, but they were needed for the combined bomber offensive. Also maybe too many short range interceptors (Spitfires) but they were protecting all UK and USA assets in UK.Nevertheless, British manufacturing output was still not able to produce all the aircraft it needed for the British Commonwealth/Empire to prosecute the war - otherwise how to explain the 31,000 aircraft imported from the USA as Lend-Lease?