Years ago, the northern perimeter road was relocated as a prelude to upgrading the aerodrome infrastructure to cater for TriStar LVPs and autolands.
Then someone realised that local topography, roads and other features would also need to be taken into account - which was deemed to be a very expensive move, given the relative infrequency of the need to work below Cat 1 limits...
There've been various warnings about the Brize ILS in recent NOTAMs, so its use below Cat 1 would be doubtful - I don't know whether the TAP shows lower than Cat 1 limits though, as these days MoD's contribution to Air Safety is to hide the milAIP behind a paywall...
