I usually get 18Mbps true at peak times?They can't properly support 20mb!! I'd rahter have 10mb on a decent infrastructure which is reliable than them pressing speeds higher and higher but not being able to support it.
Then again I might be completely wrong and they may have the network to support these speeds

and there are over 2 million of them. what's more you get what you pay for on virgin. if you buy a 4mb Cable line, its 4mb, where as on an adsl connection if you buy an 8mb line, your guess is anyones as its dependant on distance from the exchange, quality of the wire, weather conditions ect ect.
Sorry I forgot about the magic get out clause "upto 20Mb"