Dear Helen,
The fundamental difference is that in Spain the Beneficiary who inherits pays the Spanish Inheritance Tax and will receive the Spanish Tax demand, whereas in the UK the worldwide Estate receives the Tax demand from UK Tax office meaning there is no treaty on Inheritance Tax as they are Taxed to two completely separate entities.
The Beneficiaries in Spain only have 6 months to pay the Tax and unlike UK assets Spanish assets are unable to be sold first to pay the Tax.
If the worldwide assets of the UK Deceased are more then the combined £650,000 for a Husband and Wife after both of them passing then their future Beneficiaries have Inheritance Tax in the UK of 40% and also Tax back in Spain as well.