I agree with Andrew. It is pretty usual to draft a will in Spain governed by the testator national law just to make future steps simpler. Probably taking into account dates and the copernican turn in the applicable law to the matter it still made sense at the moment of the 2016 will.
Formerly the Spanish law would send you to the national law of the deceased, now the possibility of choice is there. Obviously, the testator clearly decided that UK law would govern her will and the Spanish instrument should just be viewed as convenient, all things considered. The possibility of international public order getting in the way is not plausible and things should run smoothly, out of the typical Property Registrator taking time to decide whether the title is sufficient and some other red tape usually related to our beloved jurisdiction.
Without much knowledge of the UK law I would say the Spanish will complies from a formal POV with the Spanish law- Spanish notaries are sound at that- and from a material POV, too. The lack of reference to the new legal system to admit the choice of law is irrelevant and that choice is valid, both with the former set of rules and nowadays.
Jose Garriga