.4
Is there a flight safety committee at lhr, this could be brought up at that forum.
It is human nature to push any rule to suit yourself, for every one who slows early there will be one who pushes their stabilised approach criteria. My own company has a 1000 ra stable with a caveat that allows you to continue if you're close to stable but not quite there, provided there is no doubt that you will be stable by 500 ra. The result is the line now treat 500 as the limit. No airline want/can allow the results of rushed approaches but that doesn't mean 160 to 4 is not achievable on most days, it just needs the workman to pull out the correct tool.
Changing to 5, well you've tried it, you would have to cover your a... with a second speed restriction to 4. The long and short of this topic is pilots should tell you if they cannot comply, if they don't they are disobeying an instruction and deserve to have their misdemeanour highlighted, CA939, 1261(or whatever you call it now),MOR or ASR there is plenty of choice.
No one wants to create a them and us scenario, but if you look to other areas where no tolerance is permitted; ie mentioning that fake bomb you packed in your luggage as you go through security, diversions to offload disruptive passengers, air rage in general etc. In these cases it really didn't take too long to change the mindset of the general public so you would imagine convincing fellow professionals should be a walk in the park.
The last option I throw out for consideration is that another operational message is added to the atis to remind pilots of their requirements.
These are already written in the notes for LL in the AIP and aerad guides but unfortunately the note above does say that aircraft commanders are requested to conform to low power, low drag operating techniques to the maximum extent practicable. It neglects to say conforming to the maximum extent consistent with the following speed restrictions ...
Anyway, best of luck. Personally I endeavour to not bother anybody with additional paperwork.
IN