It's illegal for the car to have been kept in Spain with a UK registration to a UK address for more than six months. So you, or the owner, could get done for that offence. If you can physically take possession of the car in Spain, which will mean insuring it, then get it back to the UK either by driving it or by ferry, that would be best. You then register it to yourself once you get it back to the UK. If it's a genuine classic more than 40 years old you should not get flagged up by the ANPR system. But on the face of it, this reads like a scam. They want you to pay for a car in England while the car is in Spain. That's a good way to lose your money.