For more information & tickets see https://www.seat61.com/ferry-to-spain.htm. This video shows a journey from Madrid Atocha to London Waterloo, by high-speed Alvia train from Madrid to Santander, then Brittany Ferries' superb ferry 'Pont-Aven' from Santander to Portsmouth, then train to London.
We travelled Madrid to Santander for just €38 each in Preferente (1st class) using a 'Mesa' (small group) fare booked at www.renfe.com. This gave us access to the Sala Club (1st class lounge) at Madrid Atocha, with free coffee, soft drinks and beer, and the fare includes a hot tray meal on board the train complete with wine, and rounded off with a brandy.
We booked the ferry at www.brittany-ferries.co.uk, in our case this worked out as £80 each with berths in a Club Class cabin with satellite TV, shower & toilet.