In a nutshell it's not used at LHR (and most other airports) because the bigger aircraft are "built" to fly a roughly 3 degree glideslope - just about the worldwide standard. A 3 degree slope for most types gives you a lowish power setting but not such a low setting that the engine is running so slowly it takes a long time to power up should you need it. Flying anything much steeper means flying the approach with the power back at or near idle and gives a high rate of descent near the ground ( especially on the heavy jets with highish approach speeds)...the guys and girls that fly into LCY are not flying heavies, and may have specially modified aircraft/software/procedures.