As far as I know …….. I told them that “they can” opt that UK will rules apply when making a will via a Notario
Am I correct ?.

The reason is because their children live in the UK and do not have the funds to pay inheritance costs should the Spanish system take place ? (This is a worry for them as the parents (residents) live in Spain & have a Spanish will)

Any legal/ actual law would be appriciated.

The so called Brussels IV clause only means that the deceased's estate will not be subject to Spanish forced heirship rules. It does not have any effect at all on Spanish inheritance tax. But some regions have very generous deductions now, so much so that children inheriting from a parent would pay no tax, Madrid and Andalucia for example.

Information on the reductions to IHT for Group I and II beneficiaries in certain regions:-

How Spain’s succession tax reforms are protecting family inheritance | Blevins Franks

And an explanation of what effect the Brussels IV clause has

Brussel IV's impact on UK succession post-Brexit

Thanks for the info & links.

Even if their children won't have to pay inheritance tax (if the parents live in one of the regions with the more favourable IHT regimes) the parents should still make them aware that they'll have legal costs to pay (not really possible for people who don't know the Spanish tax and legal systems well, or speak Spanish, to deal with an inheritance, a tax return still has to be submitted even if there's no tax to pay and then a notary has to draw up an escritura de herencia) whereas for a simple estate it's quite straightforward to apply for probate without using a solicitor in the UK. The beneficiaries will also need to get a Spanish NIE number. They may also have to pay plus valia to the local authority on the parents' property (some Ayuntamientos no longer charge plus valia on inherited properties (eg Nerja for example) but mine (Velez Malaga) still does so they should check out the position where they live.

Spot on ...... Im still dealing with the tax aspect of my mother's estate in Tenerife following her death in 2024. I had no IHT to pay but Im still in the process of reclaiming the tax withheld when I sold the property that I inherited. The other thing worth mentioning that many Notaries in Spain will ask for with a UK citizen is probate or letters of administration from the UK to ensure that there is no conflicting will or disputes in the UK. My understanding is that this is at the discretion of the notary and depends on how long the deceased had been habitually resident in Spain.