If someone dies without a valid Will in in Spain, then the deceased will have died 'intestate'.
This means that the Spanish estate of the deceased will be distributed according to the intestacy laws of of Spain. If there is a UK Will but no Spanish Will then it may be possible to deal with the Spanish "immoveable property" only under the UK Will. Its complex, its uncertain and it adds costs to your estate.
Its always better to have a UK Will alongside your Spanish Will.
Spanish intestacy laws are rigid and uncompromising, spouses are especially not well cared for and there is no room for specific gifts to children or appointing Guardians for the care of children.
It's always better to make a UK Will to compliment your Spanish Will.