Labour did the same thing too (c.f.: Jacqui Smith, et al.): so whichever party is in government they'll kowtow to the Home Office via the Home Secretary. For good, for bad, for worse.
The Home Office was described by a former Labour Home Secretary as "not fit for purpose" (and he promptly broke it up). That's the only time I can recall a Home Secretary not following the Home Office's repressive, xenophobic attitude.
New ministers undergo an induction ritual with their civil servants; they are told what polcies are favoured, and what policies are no-go. They are confronted by a succession of generals and spies, explaining tothem that the world will fall apart if they don't toe the departmental line.
I'm convinced that this induction ritual is performed most thoroughly by the Home Office. Apparently-reasonable politicians always turn into authoritarian nutters when they become Home Secretary. I assume that the process amounts to blackmail: "The Prime Minister wants repressive policies from this department, but with deniablility. You are that deniability. Rest assured that if you don't toe the departmental line, you will be fired by the PM."
I tried to watch that episode; fail. I watched the initial (unskippable) ad; when I tried to skip to 12m, I got another unskippable ad. Is there some other source for this documentary?
Yeah, and a dozen other pay-for streaming services. Thanks; no thanks. I already pay £160 per year to watch basically anything (BBC licence fee is due even if you don't watch BBC - I live in the UK), and I'm not paying extra for every other streaming service I might watch for an hour every couple of months.
(Note that I am not defending the Tories here)