Would it be legal for an Heathrow Approach controller to actually say the words "Cleared for ILS approach rwy 25 at Northolt"
Don't know if this helps, but when I was doing my PPL at Woodford in the late 90's, Manchester Radar would often position traffic to the Woodford ILS, and hand over to Woodford Tower.
The phraseology used by Manchester Radar was 'G-CD,
Woodford clears you for the ILS approach Runway 25' - This would suggest that the clearance needs to come from the unit relating to the airfield the approach is been flown in to, and not issued on the volition of the unit being worked without co-ordination.
This is purely based on the above phraseology. I am a FISO and a PPL, so other people will be more qualified to say for sure.