La previsible explicación de Amazon.uk:

In the United Kingdom, the VAT rate is 0% on a number of select items, including books, children’s clothing and safety gear. However, many items in our catalogue are subject to the UK VAT rate of 20%, by law.

However, if your delivery address is Spain then, particular VAT charges are applied to your order. During the checkout process, the UK VAT rate will be removed and Spain VAT rate will be applied, this is why there is price difference on your order.

For your reference, I have provided the detailed cost for the item Universal Classic Monsters: The Essential Collection [Blu-ray].

Item price with UK VAT (20%) = 36.00 GBP
Spain VAT rate for DVD = 21 %
Item price including Spain Vat rate = 36.30 GBP (30.00 without UK VAT + 6.30 Spain VAT)

Please remember that VAT charges only apply to orders being sent to member countries of the EU.