INHERITANCE TAX ON WORLDWIDE ESTATE OR ONLY SPANISH ASSETS?

No, in the UK it is the whole estate that is taxed, not the heirs directly. This differs to spain, as you say, where the heirs are taxed.

Because of this difference, and the lack of provision for it in the current UK-Spain double tax treaty, it raises some serious concerns about double taxation in this matter, which is why I am curious about how it is dealt with and calculated.

Even so , whatever is left to someone resident in Spain  is subject to IHT

In the UK spouses are exempt from IHT , unlike Spain

the IHT threshold in the UK will soon be a million pounds.

it is unlikely the majority of inheritees will pay any tax in UK.

as there is no double taxation treaty on IHT, it is possible that some very wealthy people will have to pay tax in UK and Spain,

However,  receiving a massive amount of unearned money can never be bad, even if yuou have to pay tax!

Camposol:

I am adding some comments below in bold green ( same text as your message below):

Even so , whatever is left to someone resident in Spain  is subject to IHT. Yes, because of fiscal/tax residency.

In the UK spouses are exempt from IHT , unlike Spain

the IHT threshold in the UK will soon be a million pounds.

it is unlikely the majority of inheritees will pay any tax in UK.

as there is no double taxation treaty on IHT, it is possible that some very wealthy people will have to pay tax in UK and Spain, They will always be able to deduct the bill paid in the Uk to the Spain one. According to an European Recommmendation ( not mandatory and not being met by Spain),  if these people were Spain residents, tax burden should never be higher than what it was if just Spain IHT tax were involved.

It is clear that a Directive on European IHT tax is necessary.

However,  receiving a massive amount of unearned money can never be bad, even if yuou have to pay tax!

As I said said, with the massive allowance in UK it is highly unlikely that most would pay any UK tax, unlike Spain, where, if you are unfortunate enough to live in Murcia, you may well have a hefty bill!