DVSA has come back to us about your situation, saying: "The driver will not be covered to drive the vehicle in Spain once their MOT expires, so they will need to arrange for their vehicle to be transported to the UK once they are able to do this. On return, they are allowed to drive from the port or terminal to an MOT station for a pre-booked test. The driver should check with the Spanish authorities what the rules are about using a non-Spanish vehicle beyond the expected period of their visit. It is possible that the driver may need to register the vehicle locally and have their equivalent of an MOT. If your tax is due to run out, register your vehicle as ‘off the road’ - you cannot renew your vehicle tax if your MOT has expired."