There is an interesting read on Spanish inheritance tax at https://www.spanishpropertyinsight.com/2016/02/08/spanish-inheritance-tax/
Unfortunately, it describes this situation (UK estate, Spanish tax-resident inheritors) as "a scenario you categorically want to avoid for your heirs at all costs. It entails for your loved ones spending greater time, money and hassle. It has no associated advantage and numerous drawbacks."
The issues seem to be (more detail on that webpage):
The Spanish taxation system starts to impose punitive charges and penalties on unpaid IHT after 6 months. But UK probate can take much longer than that to sort out.
The Spanish system taxes the recipients of inheritance, while the UK taxes the estate distributing it. Normally countries have tax-treaties to avoid such issues with "double taxation". However:
Absurdly neither the United Kingdom nor Spain have included this
matter in article two of their double taxation treaty when it affects
thousands of British citizens every year. ... For some bizarre reason
(only privy to politicians) Spain has only signed such a treaty with
the following three countries: France, Greece and Sweden. ... This
translates in practice into having to pay for inheritance tax both in
the UK and Spain.
Seems a nasty situation to be in. Personally, I'd be looking for professional advice.