Originally Posted by EverHopeful
you will not have an issue, but in case of disasters, legal issues and the like Spain will not recognise your British nationality and will not give the British consulate access whilst you are located in Spain. As regards health care, I have no idea, but given you have not lived in Spain nor paid into the system you may not have automatic access.
All a Spanish citizen had to do is enter and do everything in Spain with (preferably) their currently valid national ID card, or alternatively their Spanish passport.
In addition there are many countries that do recognise dual nationality, yet still apply the same rules re Consulate assistance etc.
All a Spanish citizen has