You do have to have a new will, cancelling out the previous one. The reason to do this is that conflicting testaments may cause problems when enforcing them. With regards to your UK will, it certainly covers your wishes for after death, as it can be legalized for use in Spain. It will however cost you more since you will have to go through the probate procedures in the UK, and then have it translated and legalized for use in Spain.

Are you following any procedure for inheritance in the UK? If you do not have a will in the UK, nor in Spain, you should follow the an intestate procedure in either country, as your mum may have been a resident of Spain and in this case, the process could be conducted via a Spanish “declaracion de herederos”, for which you will require a certificate proving what the laws of England & Wales designate you as an inheritor, in the abscence of a will. You can also do this in the UK and then legalise the document in Spain.

If your father had a Spanish will then you don’t need to go through Probate in Spain, but enforce it through a Notary Public. You will need a death certificate, details of the assets to be inherited and a few other minor documents the Notary will request for you.

The process of Grant of Probate in the UK is not an inconvenience to pursue acceptance of inheritance in Spain, so long as both are not contradictory.

From the information you have supplied to me it appears that the case would tick all boxes for a succesful challenge in Court of the Spanish will, since your Dad did not have any assets (property) outside Spain, or at least in the Uk. This last point is important as it is the basis on which Spanish law on inheritance kicks in, and as far as you are concerned, the forcible inheritors rights over the estate that cannot be taken away from you.