FWIW, BALPA's Flight Safety Group have been in discussion with the UK authorities for a number of years over this particular issue.
I understand that the history of non-ICAO standard phraseology at LHR for an ILS descent clearance, followed the opening of LCY airport.
I have heard that NATS have recently proposed some R/T changes for LHR which are still not ICAO compliant. It is unlikely that any non ICAO proposal would be supported by BALPA.
The general feeling is that LHR should comply with all ICAO procedures.