s.1 Wills Act 1963 (UK) states:
“A will shall be treated as properly executed if its execution conformed to the internal law in force in the territory where it was executed, or in the territory where, at the time of its execution or of the testator’s death, he was domiciled or had his habitual residence, or in a state which, at either of those times, he was a national.”
Whilst I note the deceased is stated to have been domiciled, and resident, in Spain in 2016, it is not clear if they were so domiciled when the will was executed. If they were domiciled and resident in Spain as at the relevant date then, provided the deceased was a national of the UK at the time of execution, the will would be valid under UK Law.
However, if they were resident and domiciled in Spain, Spain will be the deceased’s “country of habitual residence” so that, as I understand the European Succession Law, the applicable succession law will be that of Spain, which includes forced heirship. I am not sure if the deceased’s remaining connection (if any) with the UK would have allowed them to elect for the law of a UK jurisdiction to apply to their estate, and the questioner does not indicate if the will contained any such “election”, in the absence of which the Spanish succession law will apply, regardless of the terms of the will.