I remember when almost all the rail/ship/rail connections were seamless, with integrated port/station facilities. We still have that on the route from Harwich to Hook of Holland (though the later is now a metro station link, not mainline), and (give or take a few hundred yards’ walk) a rail-boat link at Newhaven … hence the importance of not losing that route.

Dieppe used to have trains on the quayside meeting the boats (some of the rails are still there; it was a bit like Weymouth used to be, with tracks along a road); but I presume that stopped partly at least because the main ferry dock moved to the other side of the harbour where there was no scope to reroute the tracks to. That crossing enabled a useful overnight London-Paris connection, with the integrated port/station on each side, and (then) direct Dieppe-Paris trains meeting the boats.

And as you say, the port stations closed in Calais and Dover. I imagine the Dover one would be difficult to reinstate, but – as you also say – there could at least be a dedicated shuttle-bus link (indeed there was for a while, years back, on both sides).

The other missing link is the route I used to use quite a lot for overnight London-Brussels journeys, via Ostend. Ostend still has an integrated terminal for the port and railway (and indeed for the trams and buses too – all highly convenient). The one snag, of course, is that Ostend no longer has any passenger ferry connections! A fast ferry or catamaran from a rail-connected port in south-east England, linking to Ostend, would be an option I’d certainly use (as a cheaper alternative to Eurostar at short notice) for London-Brussels (and onwards) journeys.

If we really achieve the switch from flights to surface travel that is desperately needed, then the Channel Tunnel will be insufficient, and then expanded – and more convenient and speedier – surface connections will be necessary.

I well remember years ago getting off a boat at, eg, Hook of Holland, or Ostend, and finding trains waiting alongside the ferry terminal with through carriages to destinations on the other side of Europe. For longer journeys incorporating a ferry, places that do still have a port/station connection (such as Ostend) are the obvious ones to develop services round.