Virgin's booking system actually informed me quite clearly that a BoJ was not allowed with this ticket. I can still see this remark when I access the booking in my account history.
Which leads to another question: as I said, without that incident I could have easily made the 0747 to London (ferry arrival right on time, smooth transfer to Hamilton Sq). Now, what if I had decided just to stay in Liverpool for another hour (e.g. to have breakfast). Technically, this would've been an illegal BoJ then, wouldn't it? Or what if I had stayed in London for, like, three hours before continuing to Gatwick?
The ticket not being an Advance, so no mandatory reservations to show at inspection, there would hardly be any way to prove the BoJ, would there?