I can only give you advice from some experience, as I was looking to do the same with an english RHD Renault 5.
You’ll need to do the headlights as “Motoring Matters” has mentioned (scrap yard). Then you’ll have to get in contact with an official engineer, who will translate the technical documents over to spanish. (150-200 euros…. maybe around low end of that figure if I remember correctly). ITV (spanish MOT) will inspect the car ($?) (...that’s as far as I’ve gotten.) Then, I assume, you’ll have to go through paper work with the DGT to get plates, register the car and insure it (I hope your Spanish is up to scratch). If you were to go that route I’d give some leeway driving under insurance so you won’t have to tow the car from A to B. If ITV finds a problem with the car you might (??) run more of a bill (RHD) but if it’s a LHD (bought in England) I wouldn’t see much problem in it passing since LHD’s are European…. anything can happen really
As I’m a penny pincher, I’d look into doing a day trip to England from Santander with the car after spending 6 months in Spain (if it’s cheaper) which will probably give you another 6 months on paper allowing you to technically wing it for the last three months? (That’s if you are definately here for 9 months.. or 12 for argument’s sake.) If you pull the day trip plan, I’d make sure you are fully insured for Europe with your company in England at all times. (Talk to them of the ins and outs?)
I’d also take into consideration if the Spanish insurance company that you choose recognizes your insurance company in England as to avoid being treated as a new driver and getting hit by maximum insurance rates. (that I can vouch from experience, although under entirely different circumstances)
If you buy in Spain, make sure that the ITV is up to date. The car being purchased must have it, by law, before it is sold.