Some thoughts on this question from a Spanish lawyer:
From the Spanish side what is needed is this: Acuerdo de Disolución y Liquidacion of the UK company as well as a document where the now dissolved company appoints this sole director as a liquidator (“liquidador”).
These set of documents must be signed before Notary public with the Apostille seal and a sworn translation to Spanish is necessary.
The problem here is that one does this at the time of winding up the company not four years after. You may have to get the company reinstated, and then follow the above.