I have had several classic cars in Spain, currently an MG Midget. Because it is registered in the UK I was able to get cheap classic car insurance for it from one of the many UK companies with a 6 month European cover. There does not seem to be any way that they know when, or how long, it has been out of the UK because it is over 40 years old and does not need taxing or MOT........
I know that Beneficial Insurance - Jayne - will cover you, give her a try.
Spanish classic car insurance does not have the use restriction which one of the replies has mentioned except for the mileage limit.
I would advise putting the car on Historic plates because Spanish classic insurance is quite cheap and you only need an ITV every 4 years and you are legal......
Richard