I seem to get about 3 - 4Mbps max download speed, which is what I got before. Maybe there is something wrong with my setup, but I have one box connected directly to the modem with 100Mbps ethernet and I have never seen it go even close to 20Mbps unfortunately.
Anecdotally, any time I've measured I've got almost bang on what I'm paying for (at several speed rates and in three different houses over several years, though all in one general area).
More scientifically the Ofcom report about the poor performance of ADSL compared to advertised speed noted that it didn't really apply to cable based systems.
http://www.ofcom.org.uk/media/features/broadbandspeedsjy
They got a table there that shows on ADSL you're getting 50% of the 8MB they advertise on average, on Virgin Media you're getting 85% of the 10MB advertised speed.
Obviously if some folk are getting 100% then other's can be on 70% and it'll still average out but I'd say it's worth getting your line checked if you're only getting 20% of what you're paying for.
I am still running one of the really old grey modems though. They sent me a new black one ages ago, but I have not activated it as yet. Maybe I should give that a try (have held back for fear of it knocking out my broadband for a few days if things go wrong!) and see if things get better.
My speed is pretty good, but I have never been able to push it toward even 10Mbps no matter how many downloads I have going.