Just as a UK MOT test is only valid on a UK registered vehicle so a Spanish ITV is only valid on a Spanish registered car.
A Spanish ITV test is a certificate of roadworthiness for a Spanish plated vehicle. It would mean that the insurance company would be happy to accept a claim in the event of an accident.
However, it has no validity on a foreign plated vehicle in terms of insurance cover and is literally not worth the paper it is printed on.
The ITV station will happily take your money but should really point out that the ITV test is only the first stage in re-registering your car and has no validity on a UK plated vehicle.
A certificate of roadworthiness is only legally valid when it is issued in the country of registration.
If you are in any doubt you should ask your insurance company to provide you with a letter stating that they are fully aware that you are living in Spain full time and that your car is UK registered with no valid MOT or road tax and that they are fully prepared to carry on providing you with comprehensive cover.
Once you have that letter, carry it with you at all times.
If your UK tax has expired then your car is no longer licensed for use on public roads in the EU.
If your UK MOT has expired then your car no longer has a valid certificate of roadworthiness.
An unlicensed car with no certificate of roadworthiness could be impounded by the police or, in the unfortunate event of a serious accident, you could find your insurance cover withdrawn by the underwriter.
