NXDOMAIN is hijacked on Virgin Media for sure, unless you are using some other DNS. I haven't noticed them injecting ads though.
>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