There should be no legal problem with this. The vast majority of FI's now have JAA licences, so they can teach across europe with no real restrictions. (apart from language of course.)
It would be better to have an FI who had experience of flying in Spain as things are bound to be a bit different from the UK.
There must be some english speaking FI's around Madrid. I know plenty of Spanish FI's in the UK and to a man their English is excellent. (Certainly putting my Spanish to shame!)