The other alternative is to buy a LHD car in the UK and import it..its not that hard when you know how (as we now do!!).
I think if you brought a Spanish reg car in the UK (or anywhere else) you would still need to goto the Jefe de Traffico to change the ownership details and pay a probably quite large fee to do this. If you buy from a garage I think they do this for you but its already in the costs or you can pay someone else to do so.
Another thing to be wary of it that a friend got a huge fine through the post Friday as she was spotted by a traffico car driving around Asturias and the in car database had her listed as not having insurance..when she had! She was not stopped (this was all dated March!!) so is having a huge stress trying to sort this out. No idea what would happen if a car disappeared out of the country for a while then reappeared, I guess it would be on this database!!!