On another point, Halton is the Deferred Facility safety site for nearby Chequers. If the PM needs security, Halton House is the first choice and its physical security reflects this.
While there are better qualified people than me on this site, if there is one thing that isn't secure it is Halton. Not one part of the estate is secure from people walking around.
OTOH, HQ Air is now better, 1 site especially. Least you can't just walk through all the buildings.