You say that sail-rail passengers are a dying breed - I would have thought that it may be popular with families with children - particularly on routes such as Portsmouth to Bilbao I would think that in the summer holidays in particular, Londoners may use the train to Portsmouth and then the ferry to, say, Bilbao or Santander. (I would think it would be cheaper than taking a car over and perhaps less stressful than flying, particularly with young children).
I have no experience of it however (both sail and rail and children).
Numbers on short sea crossings to Ireland have been hit by low cost airlines in the last decade. However, it's possible that numbers are now set to increase, or have increased recently.
On longer sea crossings, mini breaks are popular. I have travelled to Northern Spain many times on these types of offers, and thy are generally pretty well loaded. However, most people appear to park at the port, rather than arrive by train or bus.
A fair proportion also take their cars or motorbikes with them,. and drive to their destination, whether they have families or not. Personally, I cna see the attraction of having my own vehicle if I have kids, rather than shpeherding the family and luggage through the streets of Santander to get to the raiwlay station!
I have also travelled a few times on DFDS from North Shields to Ijmuiden. A couple of double decker buses are all that is usually required from Newcastle Central station, which seems to indicate that most of the passengers on the ferries are not arriving by train. Especially since there are a fleet of coaches waiting in the Netherlands!
The ferry companies are far more road orientated. This is only to be expected as far more of their business arrives by road than by rail. Look at Holyhead where rail facilities have been swept away to improve facilities for car parking and check in. The ships no longer arrive and leave from berths adjacent to the station; the priority now is all cars and lorries.
