Is this person, because being a Spanish resident, under Spanish Inheritance Law? Can the children claim for a portion of the inheritance?
The rule for application of Inheritance Law in Spain is the rule of nationality ( provision 9.8 of the Spanish Civil Code) So initially, it might seem that inheritance Law applicable here is UK Inheritance Law which does not impose limits to the free will when testating.
UK inheritance Law applies:
1.-In relation to moveable assets the law of the country of domicile of the deceased should be applied.
2.-In relation to immoveable property (Land or buildings) it states that the law of the country where the property is located should apply.
Despite this person having residency in Spain, he might have maintained his domicile in the UK ( as domicile of origin). So by vitrtue of the unity of probate, Uk Law will decide on Spanish assets too and the UK will will be perfectly valid for UK and Spanish assets.
UK Probate documents will enter the Spanish Registry in order to change ownership rights once the probate is finished.Of course IHT related to the Spanish house will have to be paid in Spain.
In this case, a Spanish independent will shoud have been convenient in order to speed up the process regarding the Spanish assets, specially as the Bank accounts are frozen till the end of the procedure.