AFAIK the RN Signal School at Portsmouth proposed a 50cm radar in 1931, but due to lack of money no work was done until 1935 after the first experiments by Watson-Watt's team and 50cm gunnery sets entered service in 1941. If the signal school had started work 3 years earlier I doubt that the 50cm set would enter service 3 years earlier, because more testing would be done instead of rushing them into service, it would be more like a year to 18 months. However, I do think that the British Army would have adopted it as a gunnery radar in place of the GL Mk I and GL Mk II of the real world, which might mean many more kills for Anti-Aircraft Command during the Blitz.