"Virgin and other cable suppliers could probably provide similar data for their connections."
They could but there isn't much point IMHO. From my experience and that of others I know who are Virgin customers, if you're in a Virgin area you pretty much get the advertised speed.
This comes down to the the type of cable used and the fact it is much newer and was always intended to provide more than just a basic audio telephony service unlike a lot of BT's copper, maybe not the 220Mbps it's now doing but still more fit for purpose than a single twisted pair phone line.
Occasionally with Virgin post install visits are required to fit attenuators or they have to tweak the power levels from their end but of the dozen people I know on Virgin in different areas, they all get the speed advertised.
Virgin's only real speed problem is contention in some heavily populated areas.