Never seen the 7.5nm previously.
This is defined in the quarterly/annual reports published by the BAA's Flight Evaluation Unit, see
Heathrow Noise: Our reporting
"Lates and Lows: During the daytime the aircraft are required to be "established" on the ILS at 7.5nm from touchdown. This equates to approximately 2500ft aal. At night the distance is extended to 10nm which equates to approximately 3000ft aal."
The AIP puts it slightly differently, referring only to height (actually AMSL, not AAL) rather than distance from the threshold, but the net effect is the same:
"Between 0600 and 2330 hours (local time) where the aircraft is approaching runway 27 (L or R) and is using the ILS in IMC or VMC it shall not descend on the glidepath below an altitude of 2500 ft (Heathrow QNH) before being established on the localizer, nor thereafter fly below the glidepath."
The ILS approach plates for Heathrow also define 7.5nm as the Final Approach Point (FAP).