UK price includes 20% VAT which when removed brings the UK price down to £ 457.50 which is $ 635 USD. SO not so different when you take into account a bit of wriggle room for currency fluctuations.
US Sellers selling into the EU are obliged to add VAT @ 20% to their prices (Uaudio do this), but some still do not - as I believe there is a transition period to allow them to comply.
You can't blame Spitfire Audio for taxes levied by the government I'm afraid. You can claim back VAT if you are VAT registered.
https://www.vatglobal.com/industry-specific-vat-guides/charging-sales-tax-in-europe