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!