Not to be needlessly contrarian, but this doesn't appear to be the case - at least for me, for any of the domains I tried:

>nslookup notarealdomainthatshouldresolve.co.uk Server: cache1.service.virginmedia.net Address: 194.168.4.100

* cache1.service.virginmedia.net can't find notarealdomainthatshouldresolve.co.uk: Non-existent domain

NXDOMAIN used to be hijacked by Virgin Media. But it certainly is not any more, unless their DNS service changes significantly across the country.