The issue of UK sellers shipping to EU buyers as it turns out is not only a massive clustersfuck (due to brexit), but also it seems that Squarespace are completely unprepared on the issue.
Yes we can assign the tax to each EU country in the backend, but as for getting an 'IOSS' number to use on the shipping label, zero chance. Where is the support for this, how can my post be the only one questioning this issue? I found a thread on the subject of IOSS on the Shopify Forum:
https://community.shopify.com/c/Shopify-Discussion/EU-Import-One-Stop-Shop-IOSS-support/td-p/1125551
And even that is full of people who have no idea what is truly going on, and I count myself as one of them.
What an utter mess. eBay issues the seller with an IOSS number, why can't SS?