120.4: I feel your pain!
I'm going to stick my neck out and say that
under normal circumstances, i.e. no tailwind, icing, etc. I can't see why any aircraft shouldn't follow the 220/180/160 regime on approach to LHR, if they use all the aerodynamic devices available to them. If they genuinely are not able (Vat>160, for instance) then they should inform ATC at the earliest opportunity.
As BOAC says, 160 to 5 isn't going to make a significant difference but 160 instead of 180 for several miles is.
If I was doing an instrument rating and didn't fly ATC instructed speeds, I would fail - simple as that. What's the problem in real life? (Quickly dons fireproof jacket
)