[cable]

There is no real technical reason as to why BT couldn't do this on the aerial feed, or why Sky or Virgin couldn't also be asked to do this. BT's multicast infrastructure is used for these channels, ITVX itself plays no part.

So it would be fair to ask why ITV was able to force BT to agree to this, what differentiates BT from Virgin in terms of being "cable TV"?

Indeed, IIRC there was fast forwarding controls on Sky+ that were never implemented. 

But perhaps they recently renewed the carriage agreement, and IP delivered gives them more control as they are also providing watch from start capability and maybe an enhanced app.