How ASB is this?
Alan Turing gets 'employed' by the government's Department of Scientific and Industrial Research at GPO research Dollis Hill to develop the machine described in his paper (published OTL in 1936, ITTL declared secret) after completing his PhD in 1938.
Thomas Harold Flowers, Tommy Flowers, is working on thermionic valve switching systems there and the overlap between their projects is supported by government funding of a valve improvement and mass production drive.
Dollis Hill is moved to an industrialised research facility and Robert Watson Watt is brought in to supervise and secretly research "The Detection of Aircraft by Radio Methods". An institute of electronic research, development, manufacture and education comes into being in early 1939 at place between Oxford and Cambridge called Bletchley. A university and science park, if you will. Promising students are drawn in and 'enlisted'.
The 300 acre site also houses the Government Code and Cypher school. While on the face of it a public research institute, Bletchley University is a secure site and entry must be preceded by signature to the Official Secrets Act and security service clearance. Occasionally media and political guests are shown the public façade of civil research and education. The architecture of the site is designed to steer the unauthorised away from the more sensitive areas.
Underground Cypher Decrypting bunkers have deeply buried teleprinter links to intercept stations dispersed across the country. Above them small pilot manufacturing processes, research labs and specialist colleges conceal the comings and goings of those that work below. Watt uses industrial/commercial secrecy as excuse to secure and compartmentalise the buildings and grounds.
15,000 researchers, students and industrial engineers work on the surface. A further 2,500 have work below (5,000 increasing to 7,500 with a wartime evening and then night shift).
The design and staffing of the institute is the point of divergence/departure.