I was just going to post a link to that video having just watched it.
If you've not clicked on the link it is titled "Should we build HS2 or Re-open old Railway lines?" and a lot of it is talking with a railway engineer called Gareth Dennis explaining the reasoning for building it (in terms that I've not heard before about increasing capacity. (The rest is more usual fare for the channel talking about features of abandoned lines around the country).
I knew we needed more capacity (I've spent enough time waiting for trains at Preston, Wigan & Warrington Bank Quay to appreciate just how full the West Coast mainline is) but hadn't heard figures on just how big the effect of moving the fast intercity trains off the current multi-purpose lines would be - basically a tripling of the capacity.