I already emailed you this in response to your geographical misgivings, but re-posting here as it seems to complement Q's post above:

"Portsmouth is one logical place to have a passenger ferry to Bilbao - which is why the modern service operates from there.

Steel rails have to be brought from the manufacturing source, in this case the steel works near to the coastal port of Sunderland (actually, the bulk of steel rail production in Britain came from Workington in Cumbria, just round the North-West coast from Carlisle, originally using locally-produced steel, later raw steel was brought overland [by rail!] from the north-east [latterly Consett, Co Durham]. Rail production now centres on Scunthorpe, Lincolnshire. The remains of what used to be the nationalised steel industry in Britain was operated after privatisation mainly by Corus, and now by the Indian-owned Tata company [http://www.tatasteelrail.com/en/products/rail_products/ ])

Steel-production in Britain historically centred on coal-fields and iron-ore deposits (so Midlands, north of England, Central Scotland and South Wales). Expense of transportation of raw materials and finished product by road/rail is one reason why the historical inland production centres became uneconomical as local resources ran out. So getting stuff to Portsmouth would not be an economic option. Sorry if that spoils your imagined logistics."

If no onward cargo from Bilbao could be found, the ship would quite likely be ordered back to London. I imagine crossing Biscay twice might well induce a thirst, which numerous establishments on the Ratcliffe Highway would be happy to assuage.

If you ever get back to London, Wilton's Music Hall is just around the corner from The Highway as it is now known. The oldest music-hall in the world, it survives as a working theatre. Not far away from The Prospect of Whitby which I think you found on a previous trip.

Ross