The couple of lines they sent me said that there is a requirement of 3 years for all jobs in the Home Office.
If that is the case, the Hiring Office should never have interviewed me, and certainly when I was discussing with them how we'd only recently moved over from the USA in the interview they should have brought that requirement up.
Same with when they made the phone call in advance of the official "provisional" offer. And then again when I spoke with them by phone after the provisional offer when they said they actually were wanting me for a full-time position, but had to get me "onboard" with the part-time one first.
I assume that the agencies of the British government know what their own security requirements are?
At this point I'm thinking of going back to applying with Universities, and forgetting the idea of doing government service in any form.