As a bus operator at LHR you have my sympathy if a/c cannot maintain assigned speed. If certain airlines are frequent culprits just send a letter to the fleet manager highlighting your concerns and the problems this can cause. Most fleets have regular newsletters and bulletins etc... So hopefully the pilots will get a gentle reminder in one of their communications.
Incidentally, as an aside we often struggle to reduce from 180 - 160 in the 319 and are reluctant to increase drag by using gear or spoilers b/c of noise. If this has happened do you have any issues with us dialling in 150kt when the gear comes down (usually at the 6nm mark) as the a/c is still reluctant to slow but it keeps the thrust at idle for longer and lowers the noise footprint? I only suggest this if we can see that we have started to catch the guy in front by struggling to slow down in the early stages of the approach.