You can't get 'temporary/short term' insurance as a vehicle here has to be continuously insured ( like the UK have recently brought in ) & therefore it is not offered.
Any Spanish company will offer insurance on A) The chassis number,
B) most will offer insurance on UK/other EU registered vehicles,
C) they then just change to the new spanish registration when completed.
Liberty Seguros is one Spanish company that does this & has the bonus of notifying the Dvla of any UK registered vehicle that it insures to ensure that you are on the UK database any time you return.