If you look at LGW 26L / 08R, LGWrunway there are two landing distances published for each runway. They depend on whether you follow the glide path to touch down, or deliberately fly below the glide path to cross the landing thresh hold at 50 feet on a 3 degree profile.
Long ago since using Jeppesen. Are you sure about this? A reference would be nice.
As far as I remember "beyond the glide slope", is simply a measurement of the distance from the glide slope antenna to the runway end. Which fit very well with the the antenna, in your example, is almost exactly 1000 feet down the runway or 50 feet crossing altitude of the threshold, being on the 3 deg glide slope.