Re: Roadworks on the routes?
It is possible for ping to work while http does not purely due to the packet size. Saw an issue on ADSL years ago when BT starting rolling out Fujitsu MSANs - no new customers on VPs over a certain size could view webpages but could ping using the default packet size. Gradually increase the packet size and you would eventually find a value that would not work (but too small to make changing MTU viable).
Virgin has has DPIMs on the network for years, since it is how traffic shaping functions. They have also had the capability and indeed have already been blocking sites (e.g. those on the IWF blacklist). The only problem I am aware of this causing thus far was the Wikipedia "Virgin Killers" incident, which was caused by all traffic to Wikipedia going through one proxy IP (the only way they could block one page without blocking the whole site)