Taking a car ferry from the UK to northern Spain is a leisurely travel option for Portugal albeit an expensive one and there are still around a 1000km to go to Lisbon.
Brittany Ferries sail twice weekly from Plymouth (March - December) making the crossing to Santander in around 24 hours.
P & O sail from Portsmouth to Bilbao in around 30-35 hours, twice weekly from March - December.
Tickets are available direct from the ferry companies or at most travel agents.
Check with your travel agent whether the ferries from Portimao to Funchal (Madeira), Portimao to Las Palmas (Gran Canaria) and Portimao to Santa Cruz (Tenerife) are still operating.
Local ferries also ply regularly between the north bank of the Tagus to the south bank. A ferry also connects Ayamonte in Spain with Vila Real de Santo Antonio in Portugal. The journey is a short crossing of the river that provides the border between the two countries here in the deep south of both countries.