I think the Virgin network not ready for this kind of speed yet due to Virgin having to upgrade all their cabinets with new components
Because of overheads the current VM config files will not give to 100Mbps of download. The 50Mbps config is actually 53Mbps so can deliver 50Mbps of download. To comply with ASA regulations VM will be upgrading their 100Mbps config files to allow 100Mbps of download (and even further screw up areas where they've actually sold it to somebody who uses it for anything other than speedtests and browsing).
Before VM launched the 100Mbps unlimited service and by doing so completely screwed up my area I could easily achieve 50Mbps from measured downloads or Speedtest.net (which actually shows peak not average speed although it is easy enough to monitor the actual download on a PC anyway). I never could from TBB but couldn't identify exactly why - I'm guessing it must have been the routing.
VM don't need to do anything more to support 100Mbps speeds in most areas. They do need to make substantial improvements to their local pipes to provide the actual bandwidth selling such speeds to heavy users will generate. Currently more and more areas are becoming badly congested just as mine did. As soon as I can move on to Infinity I'll be going. I may even risk ADSL if things get much worse but the unresolved HR DIS fault on the phone line which pushed me to cable a couple of years back puts me off the inevitable hassle of returning.
In general cable can support high speeds but not continuous high bandwidth usage - a couple of 100Mbps users can saturate the local pipe shared with 100+ other users. ADSL has problem matching cable speeds but can fairly easily provide huge bandwidth over fibre to the exchanges (or now to street cabinets). ADSL is uncontended back to the cabinet/exchange where there will be 100+ modems on a 200Mbps group of channels on cable. Upstream is even worse with only 18Mbps available to those 100+ modems. My own connection no rarely ever reaches the 1Mbps allowed by the config (I downgraded to 10Mbps as that's mostly all it can manage a lot of the time).
Just as ADSL was a kludge providing BB over a medium not designed for it cable was designed as a TV distribution system and kludging BB onto it involves compromises. Those compromises are becoming very visible as VM marketing push higher and higher headline speeds over infrastructure that simply can't economically provide the local bandwidth that is now being used.