Hi. Yes big can of worms this one. Any car on Brit plates without a tax disc is a big risk, any insurance cover would be invalid.
  Even with tax test and ins there are limitations, there is a good article on this site www.practicalspain.com  then go to the cars and driving section
 I am not sure wether you need residentia or not in order to have your car converted to Spanish registration, hopefully someone else can answer this, there are other very useful threads on the forum on this subject.
 Hope this helps
Nick.