The vehicle must be registered on the log book to you AT least six months before you leave England this will allow you to import the vehicle up to 30 days after you have been in Spain as part of your personal belongings and avoid the import tax.
Down side is you will have the car in the UK and will find it hard to drive on British roads even if you can get it insured. This leads to another problem you cannot insure the vehicle in the UK to get it here ( once you inform your insurers that you are taking the car overseas and not coming back your insurance will be worthless. Many people lie to the insurers to get the green card by saying that they are just going on holiday to Europe for a few weeks just so they have an insurance certificate in the car if they get stopped by the French or Spanish police. Fine as long as you're not involved in an accident then as soon as your UK insurers discover you are no longer resident in the UK your insurance will be null and void, and don't think they don't know the tricks don't forget you will have had a LHD car in the UK for the previous six months!.
Even if you buy a Spanish registered car in the UK it will still be subject to import tax and remember any debts the owner has clocked up in Spain like unpaid road tax or parking fines will be inherited with the car and payable once you change ownership title in Spain.
IF the car is second hand and does not have a Certificate Of Conformity then you will have to pay for a vehicle inspection(separate from ITV )at 200euros the inspector will be expecting to see all four road tyres of the same manufacture and more importantly the wheels and tyres have to comply with the manufactures original specification(meaning that if the manufactures say the car should have 15" wheels and 195/65/15" tyres then that's what you will have to have, also check the TYRE LOADING symbol on the tyre against the cars handbook unlike England if the car has been fitted with a cheap set of tyres that do not have the manufactures recommended load symbol then you will have to purchase a complete set of tyres just to get through the ITV even if the tyres you have on are brand new they must comply with the car manufactures recommended fitment.If you're unsure how to check this pop to a recognised tyre dealer and take a copy of this to show them.