@old-bloke I can't see the problem with selling an English registered vehicle to someone living in England, could get a relative or friend to act for them. If there's a problem in Portugal, then just drive it to Santander and hand it over in the ferry port.
That would just transfer the offence to Spain (see my post above quoting the EU Directive), selling is disposing. The wording of the relevant international treaties (that are adopted into EU and national laws) effectively require the owner (or one of their immediate family) who took the vehicle out of the country of origin to return it back there.
As with all laws, some people break them and go undetected, some get caught and pay the penalty (a very expensive penalty in Portugal - up to 6 times the ISV that would be due). It is up to the individual which path they choose, I just feel they should make their choice whilst being aware of the consequences.
Your suggestion re Santander would be a legal work around if the owner drove the vehicle onto the ferry and handed it over to the buyer on the ferry, but not in the ferry port itself.