Pera.. A few aircraft went down to around 1200 feet over London - some 11 miles from touchdown. Lord only knows why they did so but it was before the days of SSR height readouts so ATC was not aware. ATC normally descends traffic to intercept the ILS at around glidepath level, e.g 3000ft at 10 miles, etc. Some crews, having being told "cleared for the ILS" went down immediately instead of following the GP. There's a lot going on under the Heathrow glidepaths which is why the phraseology was changed.