I believe Scampton was fitted with a leader cable, although I was never able to confirm this. The leader cable gave the signal to align the ac with the centreline, and the receivers were vertical aerials moulded to the front fuselage immediately behind the radome.
The auto-ILS decision height was 250ft [in-line ILS] or 270ft [off-set ILS]. In my experience, the auto-ILS was pretty accurate - we used to practice them down to 150ft just for the experience [day, VMC]. However, always took a PAR for a reallllly bad weather approach. I only ever used the auto-throttle once [again for the experience]. It wasn't very good, because it tended to split the throttles - you trimmed each engine throttle setting by pressing the relight button, setting the desired throttle position then pressed the relight button to re-engage.
FV - your right, the reason why few airfields had in-line ILS was because of the lack of space at the end of most RAF runways. I was also told once that another reason was that the installation could be expensively damaged by an aircraft over-running the runway into the overshoot area [as happened at Alconbury in the mid 80's when a Canberra did a high speed abort and careered though the ILS installation!]