The runway 14 ILS glidepath at Leeds Bradford
is 3.5 degrees and has been since the ILS was commissioned more than 25 years ago. It's only ever been a Cat 1 system and that is the best that can be hoped for apparently. It is technically possible to adjust the GP to 3 degrees but the obstructions on the approach would still be there. This may not be relevant to Heathrow but it presents a wider context.