How are you going to buy a spanish registered car, on the internet? If so what does the company say is needed to buy the car?
I know there are some internet companies who offer LHD cars which are registered in Spain. The one thing to be wary of is doing a check to make sure there aren't any debts on these cars as you will be liable for them. Also if you don't have to correct paperwork then you will be unable to register it in your name.
If buying the car in UK you would have to register it with the DVLA and then register it in Spain. Would you be able to get the car insured without being the registered owner? As you are importing it then you have to pay a fee depending on the value of the car.
I don't have any personnal experience of this, only what I've read for research purposes as this is something I've considered doing. I think the import fee is cheaper if the car has been registered in UK for X amount of months. You have to have documentation of what tax you have paid and this will be taken off any tax due in Spain.
My neighbours imported their LHD car UK reg car and they had to pay a fee at the UK embassy in Alicante. Perhaps that website could provide some help?