Go to
www.opendns.com and read the info there. It's free and you don't even have to sign up.
The more people using the system, the more addresses it can store, whereas if Virgin Media don't know the address they go to another DNS server, and thus DNS lookup is slower. With over 3 billon lookups a day, I'm sure that opendne will hold more addresses than Virgin could ever think of.
PS. You can also block "Adult" websites if you have under 18's in your household.