If the car has all its paperwork you have 3 choices; 1, if you tell traffico you want to do the baja they will tell you that you will have to put the car on green temp plates which only last 60 days,not giving you much time to sort out anything this end... 2, you can by law keep your car in another country up to a max of 6 months, i've done this cos it buys me more time in the u.k, less cost with traffico, but you will have to go back to Spain to do the baja with traffico and the baja with your local ajuntamento 'council offices' also if it has had its tax paid you should get a refund from the ajuntamento...
3rd option, You can always do the baja telling traffico the car is already in another country but you will have to chance it whilst driving the car back thru Spain cos officially it no longer exists in Spain. Also check with your ins company that your car will be recognized in the u.k with Spanish ins, i am insured with Liberty Seguros and there ins i recognized in the u.k..
I think thats all? If this does'nt make any sense call me on my Spanish mobile 690757680... Cheers Mikey